GEO 120: Maps & Earth From AboveExam 1 Review SheetOctober 3, 2011

Thematic Maps: a map that is focused on a specific subject and is organized so that the subject stands about the geographical setting. (i.e. land cover, population density, vegetation zones)

a) Dot Distribution Map- a map on which point symbols are used to show variations in density across a surface

b) Choropleth Map- a mapping method in which each data collection area is assumed to be homogeneous throughout and is given a particular color lightness, color saturation, or pattern texture depending on the magnitude of the attribute.

c) Isoline Map- use lines of equal value to represent a variable with a continuous distribution over space

Cartogram- is a map in which some thematic mapping variable is substituted for land area or distance. The geometry or space of the map is distorted in order to convey the information of this alternate variable. There are two main types of cartograms: area and distance cartograms.

a. Area Cartogram- illustrates the relative sizes of the populations of the countries of the world by scaling the area of each country in proportion to its population

b. Distance Cartogram- typically used to show relative travel times and directions from vertices in a network.

Map Projections

Datum- a known and constant surface that is used to describe the location of points on the earth’s surface. They define the size and shape of the earth and provide a point of origin

a. Geodetic Datum- are established to provide positional control that supports surveying and mapping projects covering large geographic areas, such as a country, a continent or the whole world

Why Map Projections?The earth is a 3-dimensional object, and a map is a 2-dimensional

representation of the earth.

Map Projection: a systematic rendering of a graticule of lines of latitude and longitude on a flat sheet of paper.

Projection Surfaces: The mapping surface onto which the sphere or ellipsoid representing the earth is projected.

a. Plane Surface- A map projection made by projecting the earths surface onto a flat plane that touches the generating globe at a point or slices through the generating globe along a small circle; also called a Azimuthal Projection.

b. Developable Surface- is the flat surface onto which the earth’s features are projected

1) Cone-2) Cylinder-3) Planar

Light Source Location/ Perspective Planar Projectionsa. Gnomic Position- light source at the center of the generating globe.b. Stereographic Position- light source on the surface of the generating globe

at a point of tangency (or intersection)c. Orthographic- light source at infinity; all rays are parallel

Rhumb Lines- all lines of constant compass direction that are straight lines on the map

Types of Map Projections:

Conformity Preserved Mercator Maps-

o Transverse Mercator- Lambert Conformal Conic Stereographic

Equivalence Preserved Sinusoidal- scale is true along the central meridian and each parallel Albers Equal Area Conic- uses two standard parallels Goode Homesoline Projection- “orange-peel map”

Orientation and Tangency: Tangency: refers to the point(s) at which the projection surface touches the

globeo Standard line- the line of tangency between the projection surface

and the surface of the globe. (least distortion along this line) Orientation: how a surface us positioned with relation to the globe

o Normal-o Transverse- 90 degrees from normalo Oblique- anywhere from 0 to 90 degrees

Projection Types:o Tangent: projection surface touches the globeo Secant: projection surface intersect the globe

Tangency of projection surfaces Normal

o A plane- at the poleo Cylinder- along the equatoro Cone- along a parallel

Transverse (90 degrees from normal)o Plane- at the equatoro Cylinder- along a meridiano Cone- apex at equator (the vertex of the cone lines up with the

equator when turned 90 degrees either direction)

Mercator Maps

Constructed by Gerhardus Mercator in 1959. Projection is a tangent case cylindrical conformal projection

o Conformal projections shapes are always preserved in small area

Scale: the ratio between map distances and earth distances

Map scale: distance on a map/ distances on the earths surface

How do we express scale? A word statement: e.g. “1 inch to 1 mile” An arithmetic ratio or representative fraction (RF)

o E.g. 1:100,00 or 1,100,00o This unit is free

A graphic sumbol Line drawn on a map whose length

Different Scale Units

Linear Measurements Foot: originated from the idea of the length of a human foot

o Standardized to 12 inches by romans International foor/US survey foot

o Defined as approximately 1:0.3048 meters in 1959 Miles: also descended from romans (about 5000 feet)

Areal Measurements Most of these simply derived from linear measurements

o E.g. 9 square feet + 1 squared yard 1 acre + 43,560 square feet

o related to the structure of the U.S. Public land survey (PLSS)o each township contains 36 sectionso each section encompasses 1 square mile

1 square mile + 640 acres

GENERALIZATION Refers to the fact that no map can show all details of the real world in

which it represents (why?)o Map scale

Smaller scale+ more generalizationo Map purpose

Types of generalization employed by cartographerso Selection: Retaining the more important features and eliminating

the less important features (e.g. not including a pond in a map of a erie county)

o Simplification: reducing the complexity of shapes (e.g. the smoothing of shorlines, roads etc.)

o Combination: merging 2 similar features into 1 (e.g. representing a 2-lane road with 1 line)

o Locational shift: separating a cluster of features that are very close in space (e.g. a river running along side a roas

o Size exaggeration: used to improve legibility

9/22/11

Direction Determination Direction: the path along which something is moving or pointing

3 Norths True North- the northerly direction along a line of longitude

o Determined by sighting Polaris (the north star), which is directly over the north pole

o Gyrocompass- an instrument that indicates the direction of true north

Magnetic Northo Earth’s magnetic line of forceo The difference between magnetic N and true N is called magnetic

declinationo Magnetic north is constantly changing

Grid Northo The direction of the north trending grid lines on a mapo Typically does not match perfectly with the true north…(why?)o …due to the fact that actual meridians converge at the north pole

Directional Designation

Azimuth: directional designation measured in a clockwise direction from north

Orthodrome: direction of a great circle (based in true north)o Aka the ‘arc’ of the great circle (based on true north)

Rhumb line (constant azimuth): directional line that crosses each succeeding meridian at a constant angle

o Aka the loxodrmone Back azimuth: reverse of an azimuth Bearing: directional designation used in surveys (either from north or

south)o Back bearing: reverse of bearing

Land Navigation

Map Orientation: Inspection: matching up easily recognizable land features in the

landscape to the map

Position Determination: Resection: method of determining one’s location by referencing 3 known

ground featuresRoute Selection:

Involves

Water Navigation Nautical charts: special mapes used for navigation at sea Mercator Projection

o Often preffered for nautical charts because; A straight line drawn between any 2 points will give a

constant heading (azimuth) Q: What is the name of a directional line that crosses each

succeeding meridian at a constant angle? A: Rhumb line (loxodrome)

Measuring Depth” Datum: a fixed elevation used a s the starting point for a vertical survey

o Q: what is used as the datum for measuring depth in nautical charts?

o A: mean sea level (mean used because of tidal variations) Done by taking measruements at hourly intervals Takes around 18.6 years

GPS SEGMENTS 9/27/11

3 major Segments of a GPS; space segment control segment user segment

Space Segment:Original GPS constellation completed in 1994

21 active satellites 3 spare satellites

From 24 to 32 satellites as of 2008

Other Satellite Systems; GLOSNASS: Russian

o From 24 satellites (1994) to 14 (2006) Galileo: European Union

o 30 satellites (number not exact)

Compass: Chinao 35 satellites (number not exact)

IRNSS: Indiano In partnership with GLOSSNASS to restore existing system

Control Segment: Ground Stations that monitor the health of each satellite

o Clock error, satellite malfunctioning, etc. Originally consisting of 5 ground station in Colorado

o Location of the master control station

User Segment: The GPS receiver

o The satellites send a radio frequencyo The receiver tunes into that frequency

Base map is within the receiver Can receive 12 or 20 satellite signals

o More signals = better positional reading

GPS Information- GPS signals contain three pieces of information.

o Pseudo-Random code: Uniquely identifies the satellite

o Ephemeris Data: The path and position of the satellite

o Almanac Information: The current date and time

Location Determination

Distance to satellite is determined by the time delay between when the signal was sent and when it was received.

Four signals are required to triangulate position (x, y, z) and correct for clock errors.

Space trilateration

Some Sources of Error Atmospheric Interference

o Electromagnetic radiation in atmosphere interferes with radio systems

Multipath distortion;o The signal ‘bounces’ before received

Ephmeris Bias;o Ephemeris data indicates the path and position of the satelliteo Causes: solar radiation, gravitational fields