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CHAPTER 3

NAUTICAL CHARTS

CHART FUNDAMENTALS

300. Definitions

A nautical chart represents part of the spherical earthon a plane surface. It shows water depth, the shoreline ofadjacent land, prominent topographic features, aids to nav-igation, and other navigational information. It is a workarea on which the navigator plots courses, ascertains posi-tions, and views the relationship of the ship to thesurrounding area. It assists the navigator in avoiding dan-gers and arriving safely at his destination.

Originally hand-drawn on sheepskin, traditional nauti-cal charts have for generations been printed on paper.Electronic charts consisting of a digital data base and adisplay system are in use and are replacing paper chartsaboard many vessels. An electronic chart is not simply adigital version of a paper chart; it introduces a new naviga-tion methodology with capabilities and limitations verydifferent from paper charts. The electronic chart is the legalequivalent of the paper chart if it meets certain InternationalMaritime Organization specifications. See Chapter 14 for acomplete discussion of electronic charts.

Should a marine accident occur, the nautical chart inuse at the time takes on legal significance. In cases ofgrounding, collision, and other accidents, charts becomecritical records for reconstructing the event and assigningliability. Charts used in reconstructing the incident can alsohave tremendous training value.

301. Projections

Because a cartographer cannot transfer a sphere to aflat surface without distortion, he must project the surfaceof a sphere onto adevelopable surface. A developable sur-face is one that can be flattened to form a plane. Thisprocess is known aschart projection . If points on the sur-face of the sphere are projected from a single point, theprojection is said to beperspective or geometric.

As the use of electronic charts becomes increasinglywidespread, it is important to remember that the same car-tographic principles that apply to paper charts apply to theirdepiction on video screens.

302. Selecting a Projection

Each projection has certain preferable features. Hoever, as the area covered by the chart becomes smallerdifferences between various projections become lessticeable. On the largest scale chart, such as of a harborprojections are practically identical. Some desirable propties of a projection are:

1. True shape of physical features2. Correct angular relationships3. Equal area (Represents areas in proper proportio4. Constant scale values5. Great circles represented as straight lines6. Rhumb lines represented as straight lines

Some of these properties are mutually exclusive. Fexample, a single projection cannot be both conformal aequal area. Similarly, both great circles and rhumb lines canot be represented on a single projection as straight line

303. Types of Projections

The type of developable surface to which the sphericsurface is transferred determines the projection’s classifition. Further classification depends on whether thprojection is centered on the equator (equatorial), a po(polar), or some point or line between (oblique). The namof a projection indicates its type and its principal feature

Mariners most frequently use aMercator projection ,classified as acylindrical projection upon a plane, the cyl-inder tangent along the equator. Similarly, a projectiobased upon a cylinder tangent along a meridian is caltransverse (or inverse)Mercator or transverse (or in-verse)orthomorphic . The Mercator is the most commonprojection used in maritime navigation, primarily becausrhumb lines plot as straight lines.

In a simple conic projection, points on the surface ofthe earth are transferred to a tangent cone. In theLambertconformal projection, the cone intersects the earth (a secant cone) at two small circles. In apolyconic projection,a series of tangent cones is used.

In anazimuthal or zenithal projection, points on theearth are transferred directly to a plane. If the origin of th

23

24 NAUTICAL CHARTS

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projecting rays is the center of the earth, agnomonic pro-jection results; if it is the point opposite the plane’s point oftangency, astereographic projection; and if at infinity(the projecting lines being parallel to each other), anortho-graphic projection. The gnomonic, stereographic, andorthographic areperspective projections. In anazimuthalequidistant projection, which is not perspective, the scaleof distances is constant along any radial line from the pointof tangency. See Figure 303.

Cylindrical andplane projectionsare special conicalprojections, using heights infinity and zero, respectively.

A graticule is the network of latitude and longitudelines laid out in accordance with the principles of anyprojection.

304. Cylindrical Projections

If a cylinder is placed around the earth, tangent alongthe equator, and the planes of the meridians are extended,they intersect the cylinder in a number of vertical lines. SeeFigure 304. These parallel lines of projection are equidis-tant from each other, unlike the terrestrial meridians fromwhich they are derived which converge as the latitude in-creases. On the earth, parallels of latitude are perpendicularto the meridians, forming circles of progressively smallerdiameter as the latitude increases. On the cylinder they areshown perpendicular to the projected meridians, but be-cause a cylinder is everywhere of the same diameter, theprojected parallels are all the same size.

If the cylinder is cut along a vertical line (a meridian)and spread out flat, the meridians appear as equally spacedvertical lines; and the parallels appear as horizontal lines.The parallels’ relative spacing differs in the various types ofcylindrical projections.

If the cylinder is tangent along some great circle otherthan the equator, the projected pattern of latitude and longi-tude lines appears quite different from that described above,since the line of tangency and the equator no longer coin-cide. These projections are classified asoblique ortransverse projections.

305. Mercator Projection

Navigators most often use the plane conformal projectiknown as theMercator projection . The Mercator projection isnot perspective, and its parallels can be derived mathematicas well as projected geometrically. Its distinguishing featurethat both the meridians and parallels are expanded at the sratiowith increased latitude.Theexpansion isequal to thesecof the latitude, with a small correction for the ellipticity of theearth. Since the secant of 90° is infinity, the projection cannot in-clude the poles. Since the projection is conformal, expansiothe same in all directions and angles are correctly showRhumb lines appear as straight lines, the directions of whichbe measured directly on the chart. Distances can also be msured directly if the spread of latitude is small. Great circleexcept meridians and the equator, appear as curved linescave to the equator. Small areas appear in their correct shapof increased size unless they are near the equator.

306. Meridional Parts

At the equator a degree of longitude is approximate

equal in length to a degree of latitude. As the distance frothe equator increases, degrees of latitude remain apprmately the same, while degrees of longitude becom

Figure 303. Azimuthal projections: A, gnomonic; B,stereographic; C, (at infinity) orthographic.

Figure 304. A cylindrical projection.

NAUTICAL CHARTS 25

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progressively shorter. Since degrees of longitude appeareverywhere the same length in the Mercator projection, it isnecessary to increase the length of the meridians if the ex-pansion is to be equal in all directions. Thus, to maintain thecorrect proportions between degrees of latitude and degreesof longitude, the degrees of latitude must be progressivelylonger as the distance from the equator increases. This is il-lustrated in Figure 306.

The length of a meridian, increased between the equa-tor and any given latitude, expressed in minutes of arc at theequator as a unit, constitutes the number of meridional parts(M) corresponding to that latitude. Meridional parts, givenin Table 6 for every minute of latitude from the equator tothe pole, make it possible to construct a Mercator chart andto solve problems in Mercator sailing. These values are forthe WGS ellipsoid of 1984.

307. Transverse Mercator Projections

Constructing a chart using Mercator principles, butwith the cylinder tangent along a meridian, results in atransverse Mercator or transverse orthomorphic pro-

jection. The word “inverse” is used interchangeably wit“transverse.” These projections use a fictitious graticusimilar to, but offset from, the familiar network of meridi-ans and parallels. The tangent great circle is the fictitioequator. Ninety degrees from it are two fictitious poles.group of great circles through these poles and perpendicuto the tangent great circle are the fictitious meridians, wha series of circles parallel to the plane of the tangent grcircle form the fictitious parallels. The actual meridians anparallels appear as curved lines.

A straight line on the transverse or oblique Mercatoprojection makes the same angle with all fictitious meridians, but not with the terrestrial meridians. It is therefora fictitious rhumb line. Near the tangent great circle,straight line closely approximates a great circle. The prjection is most useful in this area. Since the areaminimum distortion is near a meridian, this projection iuseful for charts covering a large band of latitude and etending a relatively short distance on each side of ttangent meridian. It is sometimes used for star chashowing the evening sky at various seasons of the yeSee Figure 307.

Figure 306. A Mercator map of the world.

26 NAUTICAL CHARTS

308. Universal Transverse Mercator (UTM) Grid

The Universal Transverse Mercator (UTM) grid is amilitary grid superimposed upon a transverse Mercator grati-cule, or the representation of these grid lines upon anygraticule. This grid system and these projections are often usedfor large-scale (harbor) nautical charts and military charts.

309. Oblique Mercator Projections

A Mercator projection in which the cylinder is tangentalong a great circle other than the equator or a meridian iscalled anoblique Mercator or oblique orthomorphicprojection. See Figure 309a and Figure 309b. This projec-tion is used principally to depict an area in the near vicinityof an oblique great circle. Figure 309c, for example, showsthe great circle joining Washington and Moscow. Figure309d shows an oblique Mercator map with the great circlebetween these two centers as the tangent great circle or fic-titious equator. The limits of the chart of Figure 309c areindicated in Figure 309d. Note the large variation in scale

as the latitude changes.

Figure 307. A transverse Mercator map of the WesternHemisphere.

Figure 309a. An oblique Mercator projection.

Figure 309b. The fictitious graticule of an obliqueMercator projection.

NAUTICAL CHARTS 27

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310. Rectangular Projection

A cylindrical projection similar to the Mercator, butwith uniform spacing of the parallels, is called arectangu-lar projection . It is convenient for graphically depictinginformation where distortion is not important. The principalnavigational use of this projection is for the star chart of theAir Almanac, where positions of stars are plotted by rectan-gular coordinates representing declination (ordinate) andsidereal hour angle (abscissa). Since the meridians are par-allel, the parallels of latitude (including the equator and thepoles) are all represented by lines of equal length.

311. Conic Projections

A conic projection is produced by transferring pointsfrom the surface of the earth to a cone or series of cones.This cone is then cut along an element and spread out flat toform the chart. When the axis of the cone coincides with theaxis of the earth, then the parallels appear as arcs of circles,

and the meridians appear as either straight or curved linconverging toward the nearer pole. Limiting the area coered to that part of the cone near the surface of the ealimits distortion. A parallel along which there is no distortion is called astandard parallel. Neither the transverseconic projection, in which the axis of the cone is in thequatorial plane, nor the oblique conic projection, in whicthe axis of the cone is oblique to the plane of the equatorordinarily used for navigation. They are typically used foillustrative maps.

Using cones tangent at various parallels, a secant (tersecting) cone, or a series of cones varies the appearaand features of a conic projection.

312. Simple Conic Projection

A conic projection using a single tangent cone is asim-ple conic projection (Figure 312a). The height of the coneincreases as the latitude of the tangent parallel decreasesthe equator, the height reaches infinity and the cone b

Figure 309c. The great circle between Washington and Moscow as it appears on a Mercator map.

Figure 309d. An oblique Mercator map based upon a cylinder tangent along the great circle through WashingtoMoscow. The map includes an area 500 miles on each side of the great circle. The limits of this map are indicated

Mercator map ofFigure 309c.

28 NAUTICAL CHARTS

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comes a cylinder. At the pole, its height is zero, and thecone becomes a plane. Similar to the Mercator projection,the simple conic projection is not perspective since only themeridians are projected geometrically, each becoming anelement of the cone. When this projection is spread out flatto form a map, the meridians appear as straight lines con-verging at the apex of the cone. The standard parallel,where the cone is tangent to the earth, appears as the arc ofa circle with its center at the apex of the cone. The otherparallels are concentric circles. The distance along any me-ridian between consecutive parallels is in correct relation tothe distance on the earth, and, therefore, can be derivedmathematically. The pole is represented by a circle (Figure312b). The scale is correct along any meridian and alongthe standard parallel. All other parallels are too great inlength, with the error increasing with increased distancefrom the standard parallel. Since the scale is not the same inall directions about every point, the projection is neither aconformal nor equal-area projection. Its non-conformal na-ture is its principal disadvantage for navigation.

Since the scale is correct along the standard paralleland varies uniformly on each side, with comparatively littledistortion near the standard parallel, this projection is usefulfor mapping an area covering a large spread of longitudeand a comparatively narrow band of latitude. It was devel-

oped by Claudius Ptolemy in the second century A.D.map just such an area: the Mediterranean Sea.

Figure 312a. A simple conic projection.

Figure 312b. A simple conic map of the Northern Hemisphere.

NAUTICAL CHARTS 29

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313. Lambert Conformal Projection

The useful latitude range of the simple conic projectioncan be increased by using a secant cone intersecting the earthat two standard parallels. See Figure 313. The area between thetwo standard parallels is compressed, and that beyond is ex-panded. Such a projection is called either asecant conicorconic projection with two standard parallels.

If in such a projection the spacing of the parallels is al-tered, such that the distortion is the same along them asalong the meridians, the projection becomes conformal.This modification produces theLambert conformal pro-jection. If the chart is not carried far beyond the standardparallels, and if these are not a great distance apart, the dis-tortion over the entire chart is small.

A straight line on this projection so nearly approximates agreat circle that the two are nearly identical. Radio beacon sig-nals travel great circles; thus, they can be plotted on thisprojection without correction. This feature, gained without sac-rificing conformality, has made this projection popular foraeronautical charts because aircraft make wide use of radio aidsto navigation. Except in high latitudes, where a slightly modifiedform of this projection has been used for polar charts, it has notreplaced the Mercator projection for marine navigation.

314. Polyconic Projection

The latitude limitations of the secant conic projection canbe minimized by using a series of cones. This results in apoly-

conic projection. In this projection, each parallel is the base oa tangent cone. At the edges of the chart, the area betweenallels is expanded to eliminate gaps. The scale is correct alany parallel and along the central meridian of the projectioAlong other meridians the scale increases with increased difence of longitude from the central meridian. Parallels appeanonconcentric circles; meridians appear as curved lines cverging toward the pole and concave to the central meridian

The polyconic projection is widely used in atlases, paticularly for areas of large range in latitude and reasonablarge range in longitude, such as continents. However, sinit is not conformal, this projection is not customarily usein navigation.

315. Azimuthal Projections

If points on the earth are projected directly to a plane sface, a map is formed at once, without cutting and flattening,“developing.” This can be considered a special case of a coprojection in which the cone has zero height.

The simplest case of theazimuthal projection is one inwhich the plane is tangent at one of the poles. The meridiansstraight lines intersecting at the pole, and the parallels are ccentric circles with their common center at the pole. Thespacing depends upon the method used to transfer points fthe earth to the plane.

If the plane is tangent at some point other than a postraight lines through the point of tangency are great circleand concentric circles with their common center at the poof tangency connect points of equal distance from thpoint. Distortion, which is zero at the point of tangency, increases along any great circle through this point. Along acircle whose center is the point of tangency, the distortiois constant. The bearing of any point from the point of tagency is correctly represented. It is for this reason that theprojections are calledazimuthal. They are also calledze-nithal . Several of the common azimuthal projections aperspective.

316. Gnomonic Projection

If a plane is tangent to the earth, and points are projecgeometrically from the center of the earth, the result isgnomonic projection. See Figure 316a. Since the projection is perspective, it can be demonstrated by placing a ligat the center of a transparent terrestrial globe and holda flat surface tangent to the sphere.

In anoblique gnomonic projection the meridians ap-pear as straight lines converging toward the nearer pole. Tparallels, except the equator, appear as curves (Fig316b). As in all azimuthal projections, bearings from thpoint of tangency are correctly represented. The distanscale, however, changes rapidly. The projection is neithconformal nor equal area. Distortion is so great that shapas well as distances and areas, are very poorly represenexcept near the point of tangency.

Figure 313. A secant cone for a conic projection withtwo standard parallels.

30 NAUTICAL CHARTS

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The usefulness of this projection rests upon the fact

that any great circle appears on the map as a straight line,giving charts made on this projection the common namegreat-circle charts.

Gnomonic charts are most often used for planning thegreat-circle track between points. Points along the deter-mined track are then transferred to a Mercator projection.The great circle is then followed by following the rhumblines from one point to the next. Computer programs whichautomatically calculate great circle routes between pointsand provide latitude and longitude of corresponding rhumbline endpoints are quickly making this use of the gnomonicchart obsolete.

317. Stereographic Projection

A stereographic projection results from projectingpoints on the surface of the earth onto a tangent plane, froma point on the surface of the earth opposite the point of tan-gency (Figure 317a). This projection is also called anazimuthal orthomorphic projection .

The scale of the stereographic projection increaswith distance from the point of tangency, but it increasmore slowly than in the gnomonic projection. The steregraphic projection can show an entire hemisphere withoexcessive distortion (Figure 317b). As in other azimuth

Figure 316a. An oblique gnomonic projection.

Figure 316b. An oblique gnomonic map with point oftangency at latitude 30°N, longitude 90°W.

Figure 317a. An equatorial stereographic projection.

Figure 317b. A stereographic map of the WesternHemisphere.

NAUTICAL CHARTS 31

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projections, great circles through the point of tangency ap-pear as straight lines. Other circles such as meridians andparallels appear as either circles or arcs of circles.

The principal navigational use of the stereographicprojection is for charts of the polar regions and devices formechanical or graphical solution of the navigational trian-gle. A Universal Polar Stereographic (UPS) grid,mathematically adjusted to the graticule, is used as a refer-ence system.

318. Orthographic Projection

If terrestrial points are projected geometrically frominfinity to a tangent plane, anorthographic projection re-sults (Figure 318a). This projection is not conformal; nordoes it result in an equal area representation. Its principaluse is in navigational astronomy because it is useful for il-lustrating and solving the navigational triangle. It is alsouseful for illustrating celestial coordinates. If the plane istangent at a point on the equator, the parallels (including theequator) appear as straight lines. The meridians would ap-pear as ellipses, except that the meridian through the pointof tangency would appear as a straight line and the one 90°away would appear as a circle (Figure 318b).

319. Azimuthal Equidistant Projection

An azimuthal equidistant projection is an azimuthalprojection in which the distance scale along any great circthrough the point of tangency is constant. If a pole is thpoint of tangency, the meridians appear as straight radlines and the parallels as equally spaced concentric circIf the plane is tangent at some point other than a pole,concentric circles represent distances from the point of tagency. In this case, meridians and parallels appear as cur

The projection can be used to portray the entire earth,point 180° from the point of tangency appearing as the largeof the concentric circles. The projection is not conformaequal area, or perspective. Near the point of tangency distion is small, increasing with distance until shapes near topposite side of the earth are unrecognizable (Figure 319)

The projection is useful because it combines the thrfeatures of being azimuthal, having a constant distance scfrom the point of tangency, and permitting the entire earthbe shown on one map. Thus, if an important harbor or airpis selected as the point of tangency, the great-circle coudistance, and track from that point to any other point on tearth are quickly and accurately determined. For commucation work with the station at the point of tangency, the paof an incoming signal is at once apparent if the directionarrival has been determined and the direction to train a dirtional antenna can be determined easily. The projectionalso used for polar charts and for the star finder, No. 210

Figure 318a. An equatorial orthographic projection. Figure 318b. An orthographic map of the Western Hemisph

32 NAUTICAL CHARTS

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POLAR CHARTS

320. Polar Projections

Special consideration is given to the selection of pro-jections for polar charts because the familiar projectionsbecome special cases with unique features.

In the case of cylindrical projections in which the axis of thecylinder is parallel to the polar axis of the earth, distortion be-comes excessive and the scale changes rapidly. Such projectionscannot be carried to the poles. However, both the transverse andoblique Mercator projections are used.

Conic projections with their axes parallel to the earth’s po-lar axis are limited in their usefulness for polar charts becauseparallels of latitude extending through a full 360° of longitudeappear as arcs of circles rather than full circles. This is because acone, when cut along an element and flattened, does not extend

through a full 360° without stretching or resuming its formeconical shape. The usefulness of such projections is also limby the fact that the pole appears as an arc of a circle insteadpoint. However, by using a parallel very near the pole ashigher standard parallel, a conic projection with two standaparallels can be made. This requires little stretching to compthe circles of the parallels and eliminate that of the pole. Sucprojection, called amodified Lambert conformal or Ney’sprojection, is useful for polar charts. It is particularly familiar tothose accustomed to using the ordinary Lambert conformcharts in lower latitudes.

Azimuthal projections are in their simplest form whetangent at a pole. This is because the meridians are stralines intersecting at the pole, and parallels are concencircles with their common center at the pole. Within a fe

Figure 319. An azimuthal equidistant map of the world with the point of tangency latitude 40°N, longitude 100°W.

NAUTICAL CHARTS 33

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degrees of latitude of the pole they all look similar; howev-er, as the distance becomes greater, the spacing of theparallels becomes distinctive in each projection. In the po-lar azimuthal equidistant it is uniform; in the polarstereographic it increases with distance from the pole untilthe equator is shown at a distance from the pole equal totwice the length of the radius of the earth; in the polar gno-monic the increase is considerably greater, becominginfinity at the equator; in the polar orthographic it decreaseswith distance from the pole (Figure 320). All of these butthe last are used for polar charts.

321. Selection of a Polar Projection

The principal considerations in the choice of a suitableprojection for polar navigation are:

1. Conformality: When the projection represents an-gles correctly, the navigator can plot directly on thechart.

2. Great circle representation: Because great circles aremore useful than rhumb lines at high altitudes, the pro-jection should represent great circles as straight lines.

3. Scale variation: The projection should have a con-stant scale over the entire chart.

4. Meridian representation: The projection should showstraight meridians to facilitate plotting and gridnavigation

5. Limits: Wide limits reduce the number of projec-tions needed to a minimum.

The projections commonly used for polar charts are tmodified Lambert conformal, gnomonic, stereographiand azimuthal equidistant. All of these projections are similar near the pole. All are essentially conformal, and a grecircle on each is nearly a straight line.

As the distance from the pole increases, however, tdistinctive features of each projection become importaThe modified Lambert conformal projection is virtuallyconformal over its entire extent. The amount of its scale dtortion is comparatively little if it is carried only to about25° or 30° from the pole. Beyond this, the distortion increases rapidly. A great circle is very nearly a straight linanywhere on the chart. Distances and directions canmeasured directly on the chart in the same manner as oLambert conformal chart. However, because this projectiis not strictly conformal, and on it great circles are not eactly represented by straight lines, it is not suited for highaccurate work.

The polar gnomonic projection is the one polar projetion on which great circles are exactly straight lineHowever, a complete hemisphere cannot be represenupon a plane because the radius of 90° from the centerwould become infinity.

The polar stereographic projection is conformal over ientire extent, and a straight line closely approximates a grcircle. See Figure 321. The scale distortion is not excessfor a considerable distance from the pole, but it is greathan that of the modified Lambert conformal projection.

The polar azimuthal equidistant projection is useful foshowing a large area such as a hemisphere because the

Figure 320. Expansion of polar azimuthal projections.

Figure 321. Polar stereographic projection.

34 NAUTICAL CHARTS

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no expansion along the meridians. However, the projectionis not conformal and distances cannot be measured accu-rately in any but a north-south direction. Great circles otherthan the meridians differ somewhat from straight lines. Theequator is a circle centered at the pole.

The two projections most commonly used for polarcharts are the modified Lambert conformal and the polar ste-reographic. When a directional gyro is used as a directionalreference, the track of the craft is approximately a great cir-cle. A desirable chart is one on which a great circle isrepresented as a straight line with a constant scale and withangles correctly represented. These requirements are not metentirely by any single projection, but they are approximatedby both the modified Lambert conformal and the polar ste-reographic. The scale is more nearly constant on the former,

but the projection is not strictly conformal. The polar steregraphic is conformal, and its maximum scale variation canreduced by using a plane which intersects the earth at soparallel intermediate between the pole and the lowest palel. The portion within this standard parallel is compresseand that portion outside is expanded.

The selection of a suitable projection for use in polaregions depends upon mission requirements. These requments establish the relative importance of various featurFor a relatively small area, any of several projectionssuitable. For a large area, however, the choice is more dficult. If grid directions are to be used, it is important thaall units in related operations use charts on the same protion, with the same standard parallels, so that a single gdirection exists between any two points.

SPECIAL CHARTS

322. Plotting Sheets

Position plotting sheets are “charts” designed primarilyfor open ocean navigation, where land, visual aids to naviga-tion, and depth of water are not factors in navigation. Theyhave a latitude and longitude graticule, and they may have oneor more compass roses. The meridians are usually unlabeled,so a plotting sheet can be used for any longitude. Plottingsheets on Mercator projection are specific to latitude, and thenavigator should have enough aboard for all latitudes for hisvoyage. Plotting sheets are less expensive than charts.

A plotting sheet may be used in an emergency whencharts have been lost or destroyed. Directions on how toconstruct plotting sheets suitable for emergency purposesare given in Chapter 26, Emergency Navigation.

323. Grids

No system exists for showing the surface of the earthon a plane without distortion. Moreover, the appearance of

the surface varies with the projection and with the relatioof that surface area to the point of tangency. One may wto identify a location or area simply by alpha-numeric recangular coordinates. This is accomplished with agrid . In itsusual form this consists of two series of lines drawn perpedicularly on the chart, marked by suitable alpha-numedesignations.

A grid may use the rectangular graticule of the Merctor projection or a set of arbitrary lines on a particulaprojection. The World Geodetic Reference System(GEOREF) is a method of designating latitude and longtude by a system of letters and numbers instead ofangular measure. It is not, therefore, strictly a grid. It is usful for operations extending over a wide area. Examplesthe second type of grid are theUniversal Transverse Mer-cator (UTM) grid, the Universal Polar Stereographic(UPS)grid, and theTemporary Geographic Grid (TGG) .Since these systems are used primarily by military forcethey are sometimes called military grids.

CHART SCALES

324. Types Of Scales

Thescaleof a chart is the ratio of a given distance on thechart to the actual distance which it represents on the earth. Itmay be expressed in various ways. The most common are:

1. A simple ratio or fraction, known as therepresenta-tive fraction . For example, 1:80,000 or 1/80,000means that one unit (such as a meter) on the chartrepresents 80,000 of the same unit on the surface ofthe earth. This scale is sometimes called thenaturalor fractional scale.

2. A statementthat a given distance on the earth equaa given measure on the chart, or vice versa. For exaple, “30 miles to the inch” means that 1 inch on thchart represents 30 miles of the earth’s surface. Simlarly, “2 inches to a mile” indicates that 2 inches othe chart represent 1 mile on the earth. This is somtimes called thenumerical scale.

3. A line or bar called agraphic scalemay be drawn ata convenient place on the chart and subdivided innautical miles, meters, etc. All charts vary somewhin scale from point to point, and in some projectionthe scale is not the same in all directions about a sin

NAUTICAL CHARTS 35

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point. A single subdivided line or bar for use over anentire chart is shown only when the chart is of suchscale and projection that the scale varies a negligibleamount over the chart, usually one of about 1:75,000or larger. Since 1 minute of latitude is very nearlyequal to 1 nautical mile, the latitude scale serves as anapproximate graphic scale. On most nautical chartsthe east and west borders are subdivided to facilitatedistance measurements.

On a Mercator chart the scale varies with the latitude.This is noticeable on a chart covering a relatively large dis-tance in a north-south direction. On such a chart the borderscale near the latitude in question should be used for mea-suring distances.

Of the various methods of indicating scale, the graphi-cal method is normally available in some form on the chart.In addition, the scale is customarily stated on charts onwhich the scale does not change appreciably over the chart.

The ways of expressing the scale of a chart are readilyinterchangeable. For instance, in a nautical mile there areabout 72,913.39 inches. If the natural scale of a chart is1:80,000, one inch of the chart represents 80,000 inches ofthe earth, or a little more than a mile. To find the exactamount, divide the scale by the number of inches in a mile,or 80,000/72,913.39 = 1.097. Thus, a scale of 1:80,000 isthe same as a scale of 1.097 (or approximately 1.1) miles toan inch. Stated another way, there are: 72,913.39/80,000 =0.911 (approximately 0.9) inch to a mile. Similarly, if thescale is 60 nautical miles to an inch, the representative frac-tion is 1:(60 x 72,913.39) = 1:4,374,803.

A chart covering a relatively large area is called asmall-scale chartand one covering a relatively small areais called alarge-scale chart. Since the terms are relative,there is no sharp division between the two. Thus, a chart ofscale 1:100,000 is large scale when compared with a chart of1:1,000,000 but small scale when compared with one of1:25,000.

As scale decreases, the amount of detail which can beshown decreases also. Cartographers selectively decreasethe detail in a process calledgeneralizationwhen produc-

ing small scale charts using large scale charts as sourThe amount of detail shown depends on several factoamong them the coverage of the area at larger scales andintended use of the chart.

325. Chart Classification by Scale

Charts are constructed on many different scales, raning from about 1:2,500 to 1:14,000,000. Small-scale chacovering large areas are used for route planning and for oshore navigation. Charts of larger scale, covering smalareas, are used as the vessel approaches land. Several mods of classifying charts according to scale are usedvarious nations. The following classifications of nauticacharts are used by the National Ocean Service.

Sailing charts are the smallest scale charts used fplanning, fixing position at sea, and for plotting the deareckoning while proceeding on a long voyage. The scalegenerally smaller than 1:600,000. The shoreline and toporaphy are generalized and only offshore soundings, tprincipal navigational lights, outer buoys, and landmarvisible at considerable distances are shown.

General chartsare intended for coastwise navigatiooutside of outlying reefs and shoals. The scales range frabout 1:150,000 to 1:600,000.

Coastal chartsare intended for inshore coastwise navigation, for entering or leaving bays and harbors oconsiderable width, and for navigating large inland wateways. The scales range from about 1:50,000 to 1:150,0

Harbor charts are intended for navigation and anchorage in harbors and small waterways. The scalegenerally larger than 1:50,000.

In the classification system used by NIMA, the sailincharts are incorporated in the general charts classificat(smaller than about 1:150,000); those coast charts especuseful for approaching more confined waters (bays, harboare classified as approach charts. There is considerable olap in these designations, and the classification of a charbest determined by its use and by its relationship to othcharts of the area. The use of insets complicates the plament of charts into rigid classifications.

CHART ACCURACY

326. Factors Relating to Accuracy

The accuracy of a chart depends upon the accuracy ofthe hydrographic surveys and other data sources used tocompile it and the suitability of its scale for its intended use.

One can sometimes estimate the accuracy of a chart’ssurveys from the source notes given in the title of the chart.If the chart is based upon very old surveys, use it with cau-tion. Many early surveys were inaccurate because of thetechnological limitations of the surveyor.

The number of soundings and their spacing indicatthe completeness of the survey. Only a small fraction of tsoundings taken in a thorough survey are shown onchart, but sparse or unevenly distributed soundings indicthat the survey was probably not made in detail. See Figu326a and Figure 326b. Large blank areas or absencedepth contours generally indicate lack of soundings in tarea. Operate in an area with sparse sounding data onrequired and then only with extreme caution. Run the ecsounder continuously and operate at a reduced spe

36 NAUTICAL CHARTS

e

Figure 326a. Part of a “boat sheet,” showing the soundings obtained in a survey.

Figure 326b. Part of a nautical chart made from the boat sheet ofFigure 326a. Compare the number of soundings in thtwo figures.

NAUTICAL CHARTS 37

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Sparse sounding information does not necessarily indicatean incomplete survey. Relatively few soundings are shownwhen there is a large number of depth contours, or wherethe bottom is flat, or gently and evenly sloping. Additionalsoundings are shown when they are helpful in indicating theuneven character of a rough bottom.

Even a detailed survey may fail to locate every rock orpinnacle. In waters where they might be located, the bestmethod for finding them is a wire drag survey. Areas thathave been dragged may be indicated on the chart by limit-ing lines and green or purple tint and a note added to showthe effective depth at which the drag was operated.

Changes in bottom contours are relatively rapid in ar-eas such as entrances to harbors where there are strongcurrents or heavy surf. Similarly, there is sometimes a ten-dency for dredged channels to shoal, especially if they aresurrounded by sand or mud, and cross currents exist. Chartsoften contain notes indicating the bottom contours areknown to change rapidly.

The same detail cannot be shown on a small-scale

chart as on a large scale chart. On small-scale charts,tailed information is omitted or “generalized” in theareas covered by larger scale charts. The navigashould use the largest scale chart available for the areawhich he is operating, especially when operating in thvicinity of hazards.

Charting agencies continually evaluate both the detand the presentation of data appearing on a chart. Devement of a new navigational aid may render previous chainadequate. The development of radar, for example,quired upgrading charts which lacked the detail required freliable identification of radar targets.

After receiving a chart, the user is responsible for keeing it updated. Mariner’s reports of errors, changes, asuggestions are useful to charting agencies. Even with mern automated data collection techniques, there issubstitute for on-sight observation of hydrographic condtions by experienced mariners. This holds true especiallyless frequently traveled areas of the world.

CHART READING

327. Chart Dates

NOS charts have two dates. At the top center of thechart is the date of the first edition of the chart. In the lowerleft corner of the chart is the current edition number anddate. This date shows the latest date through whichNoticeto Mariners were applied to the chart. Any subsequentchange will be printed in theNotice to Mariners. Any notic-es which accumulate between the chart date and theannouncement date in theNotice to Marinerswill be givenwith the announcement. Comparing the dates of the firstand current editions gives an indication of how often thechart is updated. Charts of busy areas are updated more fre-quently than those of less traveled areas. This interval mayvary from 6 months to more than ten years for NOS charts.This update interval may be much longer for certain NIMAcharts in remote areas.

New editions of charts are both demand and sourcedriven. Receiving significant new information may or maynot initiate a new edition of a chart, depending on the de-mand for that chart. If it is in a sparsely-traveled area, otherpriorities may delay a new edition for several years. Con-versely, a new edition may be printed without the receipt ofsignificant new data if demand for the chart is high andstock levels are low.Notice to Marinerscorrections are al-ways included on new editions.

NIMA charts have the same two dates as the NOScharts; the current chart edition number and date is given inthe lower left corner. Certain NIMA charts are reproduc-tions of foreign charts produced under joint agreements

with a number of other countries. These charts, even thouof recent date, may be based on foreign charts of considably earlier date. Further, new editions of the foreign chawill not necessarily result in a new edition of the NIMA reproduction. In these cases, the foreign chart is the bechart to use.

328. Title Block

The chart title block should be the first thing a navigator looks at when receiving a new edition chart. ReferFigure 328. The title itself tells what area the chart coveThe chart’s scale and projection appear below the title. Tchart will give both vertical and horizontal datums and,necessary, a datum conversion note. Source notes orgrams will list the date of surveys and other charts usedcompilation.

329. Shoreline

The shoreline shown on nautical charts representsline of contact between the land and water at a selected vtical datum. In areas affected by tidal fluctuations, thisusually the mean high-water line. In confined coastal wters of diminished tidal influence, a mean water level linmay be used. The shoreline of interior waters (rivers, lakeis usually a line representing a specified elevation abov

38 NAUTICAL CHARTS

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selected datum. A shoreline is symbolized by a heavy line.A broken line indicates that the charted position is approx-imate only. The nature of the shore may be indicated.

If the low water line differs considerably from the highwater line, then a dotted line represents the low water line.If the bottom in this area is composed of mud, sand, gravelor stones, the type of material will be indicated. If the bot-tom is composed of coral or rock, then the appropriatesymbol will be used. The area alternately covered and un-covered may be shown by a tint which is usually acombination of the land and water tint.

The apparent shoreline shows the outer edge of ma-rine vegetation where that limit would appear asshoreline to the mariner. It is also used to indicate wheremarine vegetation prevents the mariner from definingthe shoreline. A light line symbolizes this shoreline. Abroken line marks the inner edge when no other symbol(such as a cliff or levee) furnishes such a limit. The com-bined land-water tint or the land tint marks the areabetween inner and outer limits.

330. Chart Symbols

Much of the information contained on charts isshown by symbols. These symbols are not shown toscale, but they indicate the correct position of the featureto which they refer. The standard symbols and abbrevia-tions used on charts published by the United States ofAmerica are shown inChart No. 1, Nautical Chart Sym-bols and Abbreviations. See Figure 330.

Electronic chart symbols are, within programming anddisplay limits, much the same as printed ones. The less ex-pensive electronic charts have less extensive symbol

libraries, and the screen’s resolution may affect the prestation detail.

Most of the symbols and abbreviations shown in U.Chart No. 1agree with recommendations of the International Hydrographic Organization (IHO). The layout iexplained in the general remarks section ofChart No. 1.

The symbols and abbreviations on any given chamay differ somewhat from those shown inChart No. 1. Inaddition, foreign charts may use different symbologWhen using a foreign chart, the navigator should haavailable theChart No. 1 from the country which pro-duced the chart.

Chart No. 1is organized according to subject mattewith each specific subject given a letter designator. Tgeneral subject areas are General, Topography, Hydrogphy, Aids and Services, and Indexes. Under each headletter designators further define subject areas, and indivual numbers refer to specific symbols.

Information inChart No. 1is arranged in columns. Thefirst column contains the IHO number code for the symbin question. The next two columns show the symbol itsein NOS and NIMA formats. If the formats are the same, thtwo columns are combined into one. The next column istext description of the symbol, term, or abbreviation. Thnext column contains the IHO standard symbol. The lacolumn shows certain symbols used on foreign reprodution charts produced by NIMA.

331. Lettering

Except on some modified reproductions of foreigcharts, cartographers have adopted certain lettering s

BALTIC SEA

GERMANY—NORTH COAST

DAHMESHÖVED TO WISMARFrom German Surveys

SOUNDINGS IN METERS

reduced to the approximate level of Mean Sea Level

HEIGHTS IN METERS ABOVE MEAN SEA LEVEL

MERCATOR PROJECTION

EUROPEAN DATUM

SCALE 1:50,000

Figure 328. A chart title block.

NAUTICAL CHARTS 39

Figure 330. Contents of U.S. Chart No. 1.

40 NAUTICAL CHARTS

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dards. Vertical type is used for features which are dry at highwater and not affected by movement of the water; slantingtype is used for underwater and floating features.

There are two important exceptions to the two generalrules listed above. Vertical type is not used to representheights above the waterline, and slanting type is not used toindicate soundings, except on metric charts. Section 332 be-low discusses the conventions for indicating soundings.

Evaluating the type of lettering used to denote a feature,one can determine whether a feature is visible at high tide.For instance, a rock might bear the title “Rock” whether ornot it extends above the surface. If the name is given in ver-tical letters, the rock constitutes a small islet; if in slantingtype, the rock constitutes a reef, covered at high water.

332. Soundings

Charts show soundings in several ways. Numbers denoteindividual soundings. These numbers may be either vertical orslanting; both may be used on the same chart, distinguishing be-tween data based upon different U.S. and foreign surveys,different datums, or smaller scale charts.

Large block letters at the top and bottom of the chartindicate the unit of measurement used for soundings.SOUNDINGS IN FATHOMS indicates soundings are infathoms or fathoms and fractions. SOUNDINGS INFATHOMS AND FEET indicates the soundings are infathoms and feet. A similar convention is followed whenthe soundings are in meters or meters and tenths.

A depth conversion scaleis placed outside the neat-line on the chart for use in converting charted depths to feet,meters, or fathoms. “No bottom” soundings are indicatedby a number with a line over the top and a dot over the line.This indicates that the spot was sounded to the depth indi-cated without reaching the bottom. Areas which have beenwire dragged are shown by a broken limiting line, and theclear effective depth is indicated, with a characteristic sym-bol under the numbers. On NIMA charts a purple or greentint is shown within the swept area.

Soundings are supplemented bydepth contours, linesconnecting points of equal depth. These lines present a pictureof the bottom. The types of lines used for various depths areshown in Section I of Chart No. 1. On some charts depth con-tours are shown in solid lines; the depth represented by eachline is shown by numbers placed in breaks in the lines, as withland contours. Solid line depth contours are derived from in-tensively developed hydrographic surveys. A broken orindefinite contour is substituted for a solid depth contourwhenever the reliability of the contour is questionable.

Depth contours are labeled with numerals in the unit ofmeasurement of the soundings. A chart presenting a moredetailed indication of the bottom configuration with fewernumerical soundings is useful when bottom contour navi-gating. Such a chart can be made only for areas which haveundergone a detailed survey

Shoal areas often are given a blue tint. Charts designed

to give maximum emphasis to the configuration of the botom show depths beyond the 100-fathom curve over tentire chart by depth contours similar to the contours shoon land areas to indicate graduations in height. Thesecalledbottom contour or bathymetric charts.

On electronic charts, a variety of other color schemes mbe used, according to the manufacturer of the system. Colorceptionstudiesarebeingused todetermine thebestpresenta

The side limits of dredged channels are indicated by bken lines. The project depth and the date of dredging,known, are shown by a statement in or along the channel. Tpossibility of silting is always present. Local authoritieshould be consulted for the controlling depth. NOS Chafrequently show controlling depths in a table, which is kecurrent by theNotice to Mariners.

The chart scale is generally too small to permit all sounings to be shown. In the selection of soundings, least depthsshown first. This conservative sounding pattern provides saty and ensures an uncluttered chart appearance. Steep chain depth may be indicated by more dense soundings in the aThe limits of shoal water indicated on the chart may be in errand nearby areas of undetected shallow water may not becluded on the chart. Given this possibility, areas where shwater is known to exist should be avoided. If the navigatmust enter an area containing shoals, he must exercise extrcaution in avoiding shallow areas which may have escapedtection. By constructing a “safety range” around known shoand ensuring his vessel does not approach the shoal any cthan the safety range, the navigator can increase his chancsuccessfully navigating through shoal water. Constant usethe echo sounder is also important.

Abbreviations listed in Section J of Chart No. 1 arused to indicate what substance forms the bottom. Tmeaning of these terms can be found in the Glossary of tvolume. While in ages past navigators might actually navgate by knowing the bottom characteristics of certain locareas, today knowing the characteristic of the bottommost important when anchoring.

333. Depths and Datums

Depths are indicated by soundings or explanatonotes. Only a small percentage of the soundings obtainea hydrographic survey can be shown on a nautical chaThe least depths are generally selected first, and a patbuilt around them to provide a representative indicationbottom relief. In shallow water, soundings may be spac0.2 to 0.4 inch apart. The spacing is gradually increasedwater deepens, until a spacing of 0.8 to 1.0 inch is reachin deeper waters offshore. Where a sufficient numbersoundings are available to permit adequate interpretatidepth curves are drawn in at selected intervals.

All depths indicated on charts are reckoned from a slected level of the water, called thesounding datum,(sometimes referred to as thereference planeto distin-guish this term from the geodetic datum). The variou

NAUTICAL CHARTS 41

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sounding datums are explained in Chapter 9, Tides and Tid-al Currents. On charts produced from U.S. surveys, thesounding datum is selected with regard to the tides of the re-gion. Depths shown are the least depths to be expectedunder average conditions. On charts compiled from foreigncharts and surveys the sounding datum is that of the originalauthority. When it is known, the sounding datum used isstated on the chart. In some cases where the chart is basedupon old surveys, particularly in areas where the range oftide is not great, the sounding datum may not be known.

For most National Ocean Service charts of the UnitedStates and Puerto Rico, the sounding datum is mean lowerlow water. Most NIMA charts are based upon mean low wa-ter, mean lower low water, or mean low water springs. Thesounding datum for charts published by other countries var-ies greatly, but is usually lower than mean low water. Oncharts of the Baltic Sea, Black Sea, the Great Lakes, and oth-er areas where tidal effects are small or without significance,the sounding datum adopted is an arbitrary height approxi-mating the mean water level.

The sounding datum of the largest scale chart of anarea is generally the same as the reference level from whichheight of tide is tabulated in the tide tables.

The chart datum is usually only an approximation ofthe actual mean value, because determination of the actualmean height usually requires a longer series of tidal obser-vations than is usually available to the cartographer. Inaddition, the heights of the tide vary over time.

Since the chart datum is generally a computed mean oraverage height at some state of the tide, the depth of waterat any particular moment may be less than shown on thechart. For example, if the chart datum is mean lower lowwater, the depth of water at lower low water will be lessthan the charted depth about as often as it is greater. A low-er depth is indicated in the tide tables by a minus sign (–).

334. Heights

The shoreline shown on charts is generally mean highwater. A light’s height is usually reckoned from mean sealevel. The heights of overhanging obstructions (bridges,power cables, etc.) are usually reckoned from mean highwater. A high water reference gives the mariner the mini-mum clearance expected.

Since heights are usually reckoned from high waterand depths from some form of low water, the reference lev-els are seldom the same. Except where the range of tide isvery large, this is of little practical significance.

335. Dangers

Dangers are shown by appropriate symbols, as indicat-ed in Section K ofChart No. 1.

A rock uncovered at mean high water may be shown asan islet. If an isolated, offlying rock is known to uncover at

the sounding datum but to be covered at high water, tchart shows the appropriate symbol for a rock and givesheight above the sounding datum. The chart can give theight one of two ways. It can use a statement such“Uncov 2 ft.,” or it can indicate the number of feet the rocprotrudes above the sounding datum, underline this valand enclose it in parentheses (i.e. (2)). A rock which doesnot uncover is shown by an enclosed figure approximatiits dimensions and filled with land tint. It may be encloseby a dotted depth curve for emphasis.

A tinted, irregular-line figure of approximately true di-mensions is used to show a detached coral reef whuncovers at the chart datum. For a coral or rocky reef whiis submerged at chart datum, the sunken rock symbol orappropriate statement is used, enclosed by a dotted or bken line if the limits have been determined.

Several different symbols mark wrecks. The nature of twreck or scale of the chart determines the correct symbolsunken wreck with less than 11 fathoms of water over it is cosidered dangerous and its symbol is surrounded by a docurve. The curve is omitted if the wreck is deeper than 11 faoms. The safe clearance over a wreck, if known, is indicatby a standard sounding number placed at the wreck. If tdepth was determined by a wire drag, the sounding is undscored by the wire drag symbol. An unsurveyed wreck ovwhich the exact depth is unknown but a safe clearance depknown is depicted with a solid line above the symbol.

Tide rips, eddies, and kelp are shown by symbol or leend. Piles, dolphins (clusters of piles), snags, and stumare shown by small circles and a label identifying the typof obstruction. If such dangers are submerged, the lett“Subm” precede the label. Fish stakes and traps are showhen known to be permanent or hazardous to navigatio

336. Aids to Navigation

Aids to navigation are shown by symbols listed in SectioP through S of Chart No. 1. Abbreviations and additional dscriptive text supplement these symbols. In order to makesymbols conspicuous, the chart shows them in size greatly exgerated relative to the scale of the chart. “Position approximacircles are used on floating aids to indicate that they have noact position because they move around their moorings. For mfloating aids, the position circle in the symbol marks the approimate location of the anchor or sinker. The actual aid maydisplaced from this location by the scope of its mooring.

The type and number of aids to navigation shown onchart and the amount of information given in their legendvaries with the scale of the chart. Smaller scale charts mhave fewer aids indicated and less information than largscale charts of the same area.

Lighthouses and other navigation lights are shownblack dots with purple disks or as black dots with purpflare symbols. The center of the dot is the position of thlight. Some modified facsimile foreign charts use a sma

42 NAUTICAL CHARTS

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star instead of a dot.On large-scale charts the legend elements of lights are

shown in the following order:

The legend for this light would appear on the chart:

Fl(2) R 10s 80m 19M “6”

As chart scale decreases, information in the legend isselectively deleted to avoid clutter. The order of deletion isusually height first, followed by period, group repetition in-terval (e.g. (2)), designation, and range. Characteristic andcolor will almost always be shown.

Small triangles mark red daybeacons; small squaresmark all others. On NIMA charts, pictorial beacons areused when the IALA buoyage system has been implement-ed. The center of the triangle marks the position of the aid.Except on Intracoastal Waterway charts and charts of statewaterways, the abbreviation “Bn” is shown beside the sym-bol, along with the appropriate abbreviation for color ifknown. For black beacons the triangle is solid black andthere is no color abbreviation. All beacon abbreviations arein vertical lettering.

Radiobeacons are indicated on the chart by a purplecircle accompanied by the appropriate abbreviation indicat-ing an ordinary radiobeacon (R Bn) or a radar beacon(Ramark or Racon, for example).

A variety of symbols, determined by both the chartingagency and the types of buoys, indicate navigation buoys.IALA buoys (see Chapter 5, Short Range Aids to Naviga-tion) in foreign areas are depicted by various styles ofsymbols with proper topmarks and colors; the position cir-cle which shows the approximate location of the sinker is atthe base of the symbol.

A mooring buoy is shown by one of several symbols asindicated in Chart No. 1. It may be labeled with a berthnumber or other information.

A buoy symbol with a horizontal line indicates thebuoy has horizontal bands. A vertical line indicates verticalstripes; crossed lines indicate a checked pattern. There is nosignificance to the angle at which the buoy symbol appearson the chart. The symbol is placed so as to avoid interfer-

ence with other features.Lighted buoys are indicated by a purple flare from th

buoy symbol or by a small purple disk centered on the psition circle.

Abbreviations for light legends, type and color obuoy, designation, and any other pertinent information gien near the symbol are in slanted type. The letter C, N, oindicates a can, nun, or spar, respectively. Other buoysassumed to be pillar buoys, except for special buoys suchspherical, barrel, etc. The number or letter designationthe buoy is given in quotation marks on NOS charts. Oother charts they may be given without quotation marksother punctuation.

Aeronautical lights included in the light lists are showby the lighthouse symbol, accompanied by the abbreviat“AERO.” The characteristics shown depend principally upothe effective range of other navigational lights in the vicinitand the usefulness of the light for marine navigation.

Directional ranges are indicated by a broken or solline. The solid line, indicating that part of the range intended for navigation, may be broken at irregular intervato avoid being drawn through soundings. That part of thrange line drawn only to guide the eye to the objects tokept in range is broken at regular intervals. The directioif given, is expressed in degrees, clockwise from trunorth.

Sound signals are indicated by the appropriate wordcapital letters (HORN, BELL, GONG, or WHIS) or an abbreviation indicating the type of sound. Sound signalsany type except submarine sound signals may be represed by three purple 45° arcs of concentric circles near the toof the aid. These are not shown if the type of signal is listeThe location of a sound signal which does not accompanvisual aid, either lighted or unlighted, is shown by a smacircle and the appropriate word in vertical block letters.

Private aids, when shown, are marked “Priv” on NOcharts. Some privately maintained unlighted fixed aids aindicated by a small circle accompanied by the wo“Marker,” or a larger circle with a dot in the center and thword “MARKER.” A privately maintained lighted aid hasa light symbol and is accompanied by the characteristand the usual indication of its private nature. Private aishould be used with caution.

A light sector is the sector or area bounded by two raand the arc of a circle in which a light is visible or in which ihas a distinctive color different from that of adjoining sectors. The limiting radii are indicated on the chart by dotteddashed lines. Sector colors are indicated by words spelledif space permits, or by abbreviations (W, R, etc.) if it doenot. Limits of light sectors and arcs of visibility as observefrom a vessel are given in the light lists, in clockwise orde

337. Land Areas

The amount of detail shown on the land areas of nauticharts depends upon the scale and the intended purpose o

Legend Example Meaning

Characteristic F1(2) group flashing; 2 flashes

Color R red

Period 10s 2 flashes in 10 seconds

Height 80m 80 meters

Range 19M 19 nautical miles

Designation “6” light number 6

NAUTICAL CHARTS 43

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chart. Contours, form lines, and shading indicate relief.Contours are lines connecting points of equal eleva-

tion. Heights are usually expressed in feet (or in meters withmeans for conversion to feet). The interval between con-tours is uniform over any one chart, except that certainintermediate contours are sometimes shown by broken line.When contours are broken, their locations are approximate.

Form lines are approximations of contours used for thepurpose of indicating relative elevations. They are used inareas where accurate information is not available in suffi-cient detail to permit exact location of contours. Elevationsof individual form lines are not indicated on the chart.

Spot elevations are generally given only for summits orfor tops of conspicuous landmarks. The heights of spot ele-vations and contours are given with reference to mean highwater when this information is available.

When there is insufficient space to show the heights ofislets or rocks, they are indicated by slanting figures en-closed in parentheses in the water area nearby.

338. Cities and Roads

Cities are shown in a generalized pattern that approxi-mates their extent and shape. Street names are generally notcharted except those along the waterfront on the largestscale charts. In general, only the main arteries and thor-oughfares or major coastal highways are shown on smallerscale charts. Occasionally, highway numbers are given.When shown, trails are indicated by a light broken line.Buildings along the waterfront or individual ones backfrom the waterfront but of special interest to the mariner areshown on large-scale charts. Special symbols from ChartNo. 1 are used for certain kinds of buildings. A single linewith cross marks indicates both single and double track rail-roads. City electric railways are usually not charted.Airports are shown on small-scale charts by symbol and onlarge-scale charts by the shape of runways. The scale of thechart determines if single or double lines show breakwatersand jetties; broken lines show the submerged portion ofthese features.

339. Landmarks

Landmarks are shown by symbols in Chart No. 1.A large circle with a dot at its center is used to indicate

that the position is precise and may be used without reserva-tion for plotting bearings. A small circle without a dot isused for landmarks not accurately located. Capital and lowercase letters are used to identify an approximate landmark:“Mon,” “Cup,” or “Dome.” The abbreviation “PA” (posi-tion approximate) may also appear. An accurate landmark isidentified by all capital type (“MON,” “CUP,” “DOME”).

When only one object of a group is charted, its name isfollowed by a descriptive legend in parenthesis, includingthe number of objects in the group, for example “(TALL-EST OF FOUR)” or “(NORTHEAST OF THREE).”

340. Miscellaneous Chart Features

A measured nautical mile indicated on a chart is accrate to within 6 feet of the correct length. Most measuremiles in the United States were made before 1959, whenUnited States adopted the International Nautical Mile. Thnew value is within 6 feet of the previous standard length6,080.20 feet. If the measured distance differs from tstandard value by more than 6 feet, the actual measuredtance is stated and the words “measured mile” are omitt

Periods after abbreviations in water areas are omittbecause these might be mistaken for rocks. Howeverlower case i or j is dotted.

Commercial radio broadcasting stations are showncharts when they are of value to the mariner either as lanmarks or sources of direction-finding bearings.

Lines of demarcation between the areas in which intenational and inland navigation rules apply are shown onwhen they cannot be adequately described in notes onchart.

Compass roses are placed at convenient locationsMercator charts to facilitate the plotting of bearings ancourses. The outer circle is graduated in degrees with zat true north. The inner circle indicates magnetic north.

On many NIMA charts magnetic variation is given tothe nearest 1' by notes in the centers of compass rosesannual change is given to the nearest 1' to permit correctof the given value at a later date. On NOS charts, variatiis to the nearest 15', updated at each new edition if ovthree years old. The current practice of NIMA is to give thmagnetic variation to the nearest 1', but the magnetic infmation on new editions is only updated to conform with thlatest five year epoch. Whenever a chart is reprinted, tmagnetic information is updated to the latest epoch. Osome smaller scale charts, the variation is given by isogolines connecting points of equal variation; usually a seprate line represents each degree of variation. The linezero variation is called the agonic line. Many plans and isets show neither compass roses nor isogonic lines,indicate magnetic information by note. A local magnetdisturbance of sufficient force to cause noticeable defletion of the magnetic compass, called local attraction,indicated by a note on the chart.

Currents are sometimes shown on charts with arrogiving the directions and figures showing speeds. Theformation refers to the usual or average conditionAccording to tides and weather, conditions at any givetime may differ considerably from those shown.

Review chart notes carefully because they provide important information. Several types of notes are used. Thoin the margin give such information as chart number, pulication notes, and identification of adjoining charts. Notein connection with the chart title include information onscale, sources of data, tidal information, soundings, acautions. Another class of notes covers such topics as lomagnetic disturbance, controlling depths of channels, h

44 NAUTICAL CHARTS

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ards to navigation, and anchorages.A datum note will show the geodetic datum of the chart

(Do not confuse with the sounding datum. See Chapter 2,Geodesy and Datums in Navigation.) It may also containinstructions on plotting positions from the WGS 84 or NAD83 datums on the chart if such a conversion is needed.

Anchorage areas are labeled with a variety of magenta,black, or green lines depending on the status of the area.Anchorage berths are shown as purple circles, with thenumber or letter assigned to the berth inscribed within thecircle. Caution notes are sometimes shown when there arespecific anchoring regulations.

Spoil areas are shown within short broken black lines.Spoil areas are tinted blue on NOS charts and labeled.These areas contain no soundings and should be avoided.

Firing and bombing practice areas in the United Statesterritorial and adjacent waters are shown on NOS and NIMAcharts of the same area and comparable scale.

Danger areas established for short periods of time arenot charted but are announced locally. Most military com-mands charged with supervision of gunnery and missilefiring areas promulgate a weekly schedule listing activated

danger areas. This schedule is subjected to frequent chathe mariner should always ensure he has the latest scheprior to proceeding into a gunnery or missile firing areDanger areas in effect for longer periods are published inNotice to Mariners. Any aid to navigation established tomark a danger area or a fixed or floating target is showncharts.

Traffic separation schemes are shown on standard naucharts of scale 1:600,000 and larger and are printed in mag

A logarithmic time-speed-distance nomogram with aexplanation of its application is shown on harbor charts.

Tidal information boxes are shown on charts of scal1:200,000 and larger for NOS charts, and various scalesDMA charts, according to the source. See Figure 340a.

Tabulations of controlling depths are shown on somNational Ocean Service harbor and coastal charts. Seeure 340b.

Study Chart No. 1 thoroughly to become familiar witall the symbols used to depict the wide variety of featuron nautical charts.

TIDAL INFORMATION

PlacePosition

Height above datum of soundings

Mean High Water Mean Low Water

N. Lat. E. Long. Higher Lower Lower Higher

meters meters meters meters

Olongapo . . . . . . 14˚49' 120˚17' . . . 0.9 . . . . . . 0.4 . . . . . . 0.0 . . . . . . 0.3 . . .

Figure 340a. Tidal box.

NANTUCKET HARBOR

Tabulated from surveys by the Corps of Engineers - report of June 1972and surveys of Nov. 1971

Controlling depths in channels entering fromseaward in feet at Mean Low Water Project Dimensions

Name of ChannelLeft

outsidequarter

Middlehalf of

channel

Rightoutsidequarter

Date ofSurvey Width (feet)

Length(naut.miles)

DepthM. L. W.

(feet)

Entrance Channel 11.1 15.0 15.0 11 - 71 300 1.2 15

Note.-The Corps of Engineers should be consulted for changing conditions subsequent to the above.

Figure 340b. Tabulations of controlling depths.

NAUTICAL CHARTS 45

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REPRODUCTIONS OF FOREIGN CHARTS

341. Modified Facsimiles

Modified facsimile charts are modified reproductionsof foreign charts produced in accordance with bilateral in-ternational agreements. These reproductions provide themariner with up-to-date charts of foreign waters. Modifiedfacsimile charts published by NIMA are, in general, repro-duced with minimal changes, as listed below:

1. The original name of the chart may be removed andreplaced by an anglicized version.

2. English language equivalents of names and termson the original chart are printed in a suitable glos-sary on the reproduction, as appropriate.

3. All hydrographic information, except bottom char-acteristics, is shown as depicted on the original chart.

4. Bottom characteristics are as depicted in Chart No.1, or as on the original with a glossary.

5. The unit of measurement used for soundings isshown in block letters outside the upper and lower

neatlines.6. A scale for converting charted depth to feet, mete

or fathoms is added.7. Blue tint is shown from a significant depth curve t

the shoreline.8. Blue tint is added to all dangers enclosed by a do

ted danger curve, dangerous wrecks, foul areaobstructions, rocks awash, sunken rocks, and swwrecks.

9. Caution notes are shown in purple and encloseda box.

10. Restricted, danger, and prohibited areas are usuoutlined in purple and labeled appropriately.

11. Traffic separation schemes are shown in purple.12. A note on traffic separation schemes, printed

black, is added to the chart.13. Wire dragged (swept) areas are shown in purple

green.14. Corrections are provided to shift the horizontal d

tum to the World Geodetic System (1984).

INTERNATIONAL CHARTS

342. International Chart Standards

The need for mariners and chart makers to understandand use nautical charts of different nations became increas-ingly apparent as the maritime nations of the worlddeveloped their own establishments for the compilation andpublication of nautical charts from hydrographic surveys.Representatives of twenty-two nations formed a Hydro-graphic Conference in London in 1919. That conferenceresulted in the establishment of theInternational Hydro-graphic Bureau (IHB) in Monaco in 1921. Today, theIHB’s successor, theInternational Hydrographic Orga-nization (IHO) continues to provide internationalstandards for the cartographers of its member nations. (SeeChapter 1, Introduction to Marine Navigation, for a de-scription of the IHO.)

Recognizing the considerable duplication of effort bmember states, the IHO in 1967 moved to introduce the fiinternational chart . It formed a committee of six memberstates to formulate specifications for two series of interntional charts. Eighty-three small-scale charts weapproved; responsibility for compiling these charts has susequently been accepted by the member statHydrographic Offices.

Once a Member State publishes an international chareproduction material is made available to any other Member State which may wish to print the chart for its owpurposes.

International charts can be identified by the letters INbefore the chart number and the International HydrograpOrganization seal in addition to other national seals whimay appear.

CHART NUMBERING

343. The Chart Numbering System

NIMA and NOS use asystem in which numbers areassigned in accordance with both the scale and geo-graphical area of coverage of a chart. With the exceptionof certain charts produced for military use only, one- tofive-digit numbers are used. With the exception of one-digit numbers, the first digit identifies the area; the num-ber of digits establishes the scale range. The one-digitnumbers are used for certain products in the chart system

which are not actually charts.

Number of Digits Scale

1 No Scale2 1:9 million and smaller3 1:2 million to 1:9 million4 Special Purpose

5 1:2 million and larger

46 NAUTICAL CHARTS

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Two- and three-digit numbers are assigned to thosesmall-scale charts which depict a major portion of anocean basin or a large area. The first digit identifies theapplicable ocean basin. See Figure 343a. Two-digitnumbers are used for charts of scale 1:9,000,000 andsmaller. Three-digit numbers are used for charts ofscale 1:2,000,000 to 1:9,000,000.

Due to the limited sizes of certain ocean basins, nocharts for navigational use at scales of 1:9,000,000 andsmaller are published to cover these basins. The other-wise unused two-digit numbers (30 to 49 and 70 to 79)are assigned to special world charts.

One exception to the scale range criteria for three-digit numbers is the use of three-digit numbers for a se-ries of position plotting sheets. They are of larger scalethan 1:2,000,000 because they have application inocean basins and can be used in all longitudes.

Four-digit numbers are used for non-navigationaland special purpose charts, such as chart 5090,Maneu-vering Board.

Five-digit numbers are assigned to those charts ofscale 1:2,000,000 and larger that cover portions of thecoastline rather than significant portions of ocean ba-sins. These charts are based on the regions of thenautical chart index. See Figure 343b.

The first of the five digits indicates the region; thesecond digit indicates the subregion; the last three dig-its indicate the geographical sequence of the chartwithin the subregion. Many numbers have been left un-used so that any future charts may be placed in theirproper geographical sequence.

In order to establish a logical numbering system

within the geographical subregions (for the 1:2,000,00and larger-scale charts), a worldwide skeleton framwork of coastal charts was laid out at a scale 1:250,00This series was used as basic coverage except in arwhere a coordinated series at about this scale alreaexisted (such as the coast of Norway where a coordined series of 1:200,000 charts was available).

Within each region, the geographical subregionare numbered counterclockwise around the continenand within each subregion the basic series also is nubered counterclockwise around the continents. Tbasic coverage is assigned generally every 20th digexcept that the first 40 numbers in each subregion areserved for smaller-scale coverage. Charts with scalarger than the basic coverage are assigned one of19 numbers following the number assigned to the shewithin which it falls. Figure 343c shows the numberinsequence in Iceland. Note the sequence of numbaround the coast, the direction of numbering, and tnumbering of larger scale charts within the limits osmaller scales.

Five-digit numbers are also assigned to the chaproduced by other hydrographic offices. This numbeing system is applied to foreign charts so that they cbe filed in logical sequence with the charts produced bthe National Imagery and Mapping Agency and the Ntional Ocean Service.

Certain exceptions to the standard numbering systhave been made for charts intended for the military.Bottomcontour charts depict parts of ocean basins. They are identiwith a letter plus four digits according to a scheme best shoin the catalog, and are not available to civilian navigato

Figure 343a. Ocean basins with region numbers.

NA

UT

ICA

L CH

AR

TS

47

Figure 343b. Regions and subregions of the nautical chart index.

48N

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RT

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umbering system.

Figure 343c. Chart coverage of Iceland, illustrating the sequence and direction of the U.S. chart n

NAUTICAL CHARTS 49

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Combat charts have 6-digit numbers beginning with an “8.”Neither is available to civilian navigators.

344. Catalogs and Stock Numbers

Chart catalogs provide information regarding not onlychart coverage, but also a variety of special purpose chartsand publications of interest. Keep a corrected chart catalogaboard ship for review by the navigator. The NIMA catalogcontains operating area charts and other special products notavailable for civilian use, but does not contain any classifiedlistings. The NOS catalogs contain all unclassified civilian-

use NOS and NIMA charts. Military navigators receive thenautical charts and publications automatically; civilian navgators purchase them from chart sales agents.

The stock number and bar code are generally foundthe lower left corner of a NIMA chart, and in the lower righcorner of an NOS chart. The first two digits of the stocnumber refer to the region and subregion. These are flowed by three letters, the first of which refers to thportfolio to which the chart belongs; the second two denothe type of chart: CO for coastal, HA for harbor and approach, and OA for military operating area charts. The lafive digits are the actual chart number.

USING CHARTS

345. Preliminary Steps

Before using a new edition of a chart, verify its an-nouncement in theNotice to Marinersand correct it with allapplicable corrections. Read all the chart’s notes; thereshould be no question about the meanings of symbols or theunits in which depths are given. Since the latitude and lon-gitude scales differ considerably on various charts,carefully note those on the chart to be used.

Place additional information on the chart as required.Arcs of circles might be drawn around navigational lightsto indicate the limit of visibility at the height of eye of anobserver on the bridge. Notes regarding other informationfrom the light lists, tide tables, tidal current tables, and sail-ing directions might prove helpful.

346. Maintaining Charts

A mariner navigating on an uncorrected chart is courtingdisaster. The chart’s print date reflects the latestNotice toMariners used to update the chart; responsibility for main-taining it after this date lies with the user. The weeklyNoticeto Mariners contains information needed for maintainingcharts. Radio broadcasts give advance notice of urgent cor-rections. LocalNotice to Marinersshould be consulted forinshore areas. The navigator must develop a system to keeptrack of chart corrections and to ensure that the chart he is us-ing is updated with the latest correction. A convenient way ofkeeping this record is with aChart/Publication CorrectionRecord Cardsystem. Using this system, the navigator doesnot immediately update every chart in his portfolio when hereceives theNotice to Mariners. Instead, he constructs a cardfor every chart in his portfolio and notes the correction onthis card. When the time comes to use the chart, he pulls thechart and chart’s card, and he makes the indicated correctionson the chart. This system ensures that every chart is properlycorrected prior to use.

A Summary of Corrections, containing a cumulativelisting of previously publishedNotice to Marinerscorrec-tions, is published annually in 5 volumes by NIMA. Thus,to fully correct a chart whose edition date is several years

old, the navigator needs only the Summary of Correctiofor that region and the notices from that Summary forwarhe does not need to obtain notices all the way back toedition date. See Chapter 4, Nautical Publications, for a dscription of theSummaries andNotice to Mariners.

When a new edition of a chart is published, it is nomally furnished automatically to U.S. Government vesseIt should not be used until it is announced as ready for uin theNotice to Mariners. Until that time, corrections in theNotice apply to the old edition and should not be appliedthe new one. When it is announced, a new edition of a chreplaces an older one.

Commercial users and others who don’t automaticareceive new editions should obtain new editions from thesales agent. Occasionally, charts may be received or pchased several weeks in advance of their announcementheNotice to Mariners. This is usually due to extensive rescheming of a chart region and the need to announce groof charts together to avoid lapses in coverage. The maribears the responsibility for ensuring that his charts arecurrent edition. The fact that a new edition has been copiled and published often indicates that there have beextensive changes that cannot be made by hand correcti

347. Using and Stowing Charts

Use and stow charts carefully. This is especially truwith digital charts contained on electronic media. Keep otical and magnetic media containing chart data out of tsun, inside dust covers, and away from magnetic influenes. Placing a disk in an inhospitable environment mdestroy the data.

Make permanent corrections to paper charts in inkthat they will not be inadvertently erased. Pencil in all othmarkings so that they can be easily erased without daming the chart. Lay out and label tracks on chartsfrequently-traveled ports in ink. Draw lines and labels nlarger than necessary. Do not obscure sounding data or oer information when labeling a chart. When a voyagecompleted, carefully erase the charts unless there has ba grounding or collision. In this case, preserve the cha

50 NAUTICAL CHARTS

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without change because they will play a critical role in theinvestigation.

When not in use, stow charts flat in their proper portfo-lio. Minimize their folding and properly index them foreasy retrieval.

348. Chart Lighting

Mariners often work in a red light environment be-cause red light is least disturbing to night adapted vision.Such lighting seriously affects the appearance of a chart.Before using a chart in red light, test the effect red light hason its markings. Do not outline or otherwise indicate navi-gational hazards in red pencil because red markingsdisappear under red light.

349. Small-Craft Charts

NOS publishes a series of small craft charts sometimes

called “strip charts.” These charts depict segments of tAtlantic Intracoastal Waterway, the Gulf Intracoastal Waterway, and other inland routes used by yachtsmefishermen, and small commercial vessels for coastal travThey are not “north-up” in presentation, but are alignewith the waterway they depict, whatever its orientation iMost often they are used as a piloting aid for “eyeball” naigation and placed “course-up” in front of the helmsmabecause the routes they show are too confined for takand plotting fixes.

Although NOS small-craft charts are designed primrily for use aboard yachts, fishing vessels and other smcraft, these charts, at scales of 1:80,000 and larger, aresome cases the only charts available depicting inland wters transited by large vessels. In other cases the small-ccharts may provide a better presentation of navigationhazards than the standard nautical chart because of bescale and more detail. Therefore, navigators should uthese charts in areas where they provide the best covera