24
Digitizing Soil Maps with TNTmips ® Introduction to S O I L M A P P I N G

Tutorial: Digitizing Soil Maps - MicroImages, Inc. · Digitizing Soil Maps The task of digitizing soil maps is accomplished in TNTmips by using a suite of separate processes. At the

  • Upload
    others

  • View
    19

  • Download
    0

Embed Size (px)

Citation preview

Digitizing SoilMaps

with

TNTmips®

Introduction toSOIL

MAPPING

page 2

Digitizing Soil Maps

Before You Start

It may be difficult to identify the important points in some illustrations withouta color copy of this booklet. You can print or read this booklet in color fromMicroImages’ web site. The web site is also your source for the newesttutorial booklets on other topics. You can download an installation guide,sample data, and the latest version of TNTlite:

http://www.microimages.com

This booklet takes you through the sequence of processes in TNTmips that areused to digitize soil maps: from a paper map sheet to a vector object with attachedsoil attribute tables. This booklet assumes that you are an experienced TNTmipsuser. At the minimum, you should have completed the exercises in the associatedtutorials Displaying Geospatial Data, Mosaicking Raster Geodata,Georeferencing, and Editing Vector Geodata. The exercises in those tutorialsintroduce essential concepts and skills that should be second nature to you bythe time you begin this sample project.

Sample Data The exercises in this booklet use sample data that is distributedwith the TNT products. If you do not have access to a TNT products CD, youcan download the data from MicroImages’ web site: www.microimages.com. Inparticular, this booklet uses objects in the SOIL51 Project File in the SOILMAP folder.

More Documentation The exercises in this booklet use several TNTmips pro-cesses with no attempt at comprehensive documentation. Consult the TNT Ref-erence Manual for more information.

TNTmips® and TNTlite® TNTmips (The Map and Image Processing System)comes in two versions: The professional version of TNTmips, and the free TNTliteversion. Both versions run exactly the same code from the TNT products CD-ROMs and have exactly the same features. If you did not purchase the profes-sional version (which requires a software license key), then TNTmips operates inTNTlite mode, which limits the size of your project materials.

The exercises in Part One of this booklet can be completed in TNTlite. A largerproject, suitable for the professional version of TNTmips is treated in Part Two.None of the exercises can be performed in TNTview. TNTedit offers only theediting tools for cleaning and repairing auto-trace conversion artifacts.

Keith Ghormley, 26 April 2005

page 3

Digitizing Soil Maps

The task of digitizing soil maps is accomplished inTNTmips by using a suite of separate processes. Atthe heart of the effort is the Auto-Trace process,which converts grayscale raster objects (in particu-lar, scanned soil sheets) into vector objects by thin-ning and tracing line features. The Auto-Trace pro-cess (Process / Convert / Raster to Vector /Auto-Trace) provides a number of uniqueand powerful conversion filters which aredesigned to identify and delete unwantedline features during the conversion process. In par-ticular, the filters can remove soil labels and drain-age lines which, if not removed by filters, must beremoved in tedious editing sessions after conver-sion.

The main part of this booklet treats the use of theAuto-Trace process and the control settings for theline filters. Other processes, used to scan and pre-pare the soil sheets, and after conversion, to dressup the soil map vector object are touched on in laterexercises.

Digitizing Soil Maps

PART ONE of this booklet(pages 3 - 15) describesthe simple process ofdigitizing a single sectionfrom one soil sheet.

PART TWO (pages 16-23)treats the other processesthat would be used in alarger, multi-sheet digitiza-tion project.

A single section, bounded bycounty roads, has been scanned,georeferenced, auto-traced, edited,assigned attributes and styles, anddisplayed with automatic labels.The exercises in Part One of thisbook use Section 30 from Sheet 51of the Lancaster County SoilSurvey for Nebraska. This sectionoffers a number of typicaldigitization problems: a crossingrailroad, solid drainage lines,printed labels and annotations,some dark background imagevalues, and complex line intersec-tions. The following exercisesemphasize the use of the filter toolsin the TNTmips Auto-Trace processto minimize the number of un-wanted artifacts that are convertedto vector elements.

Part One: Digitize One Section

If you are starting with asoil map that wasvectorized by some otherproduct, you may be facedwith unwanted conversionartifacts that would havebeen removed by the Auto-

Trace filters inTNTmips. You canapply vectorediting filters in theTNT Spatial Data

Editor (TNTedit, or TNTmipsEdit / Spatial Data)

page 4

Digitizing Soil MapsPart One: Digitize One Section

The Auto-Trace process finds line features in ascanned map image and traces them into vector lineelements. Soil maps are commonly distributed asgrayscale airphotos with annotations and soil inter-pretations drawn in black lines. By thresholdingand filtering the image, the soil units can be auto-matically converted into polygon elements in a vec-tor object.

Launch the Auto-Trace process by selecting Auto-Trace from the Process / Convert / Raster to Vectormenu. TNTmips opens the Raster Thresholding andTracing window, a Sample Results view window, anda standard Select Object dialog. Select theSHEET51SECT30 object from the SOIL51 Project File.When you complete the selection, the process opensanother view window that displays the raster ob-ject. Select Restore Settings from the File menu andthen choose SECT30.INI (a prepared settings file thatcontains the control values illustrated here).

STEPSselect Auto-Trace fromProcess / Convert /Raster to Vectorselect SOIL51 /SHEET51SECT30choose File / RestoreSettings and selectSECT30.INI

turn off all the filtertoggle buttons

Applying filters helps remove unwanted artifacts. The color of the line elements in theSample Results window shows which artifacts are deleted by each filter. As youadjust the filters, you will see the effect of the new values. In this illustration, the redlines survive, while the yellow,orange, and blue lines are dropped.

SHEET51SECT30 is a rasterobject that was extractedfrom the scan of sheet 51from the Lancaster CountySoil Survey. The soil sheetwas scanned at 300 dpi.The extracted 1024 x 1024section was sampled to512 x 512, which retainssufficient resolution forthese sample exercises.

The Auto-Trace Process

Turn off the filter togglebuttons to prepare forthe next exercise.

page 5

Digitizing Soil Maps Part One: Digitize One Section

Adjust ThresholdThe initial threshold value provided from SECT30.INI

is 45. That means that in the 8-bit grayscaleSHEET51SECT30 raster object, all cell values from 0 to45 would be traced as line features, while all cell val-ues from 46 - 255 are discarded. In the Sample Re-sults window illustrated here, the line features showas black, and all other image values show as white.(You can change the background / traced line colorsby clicking the Colors icon button in the SampleResults window.)

As with all the controls, you can change the thresh-old value either by typing in a new value, or by drag-ging the slider. Try dragging the slider all the way tothe right end of the trough: the entire sample win-dow turns black. If you drag the slider all the way tothe left end, the entire sample window turns white.Select several values between 30 and 60. Your goalis to choose a value low enough to minimize back-ground noise, and high enough to keep most linefeatures connected.

STEPSobserve the effect ofthe threshold valueloaded from SECT30.INI

change the thresholdvalue to see the effectof other settingsclick [Test] to view a linetrace of the thresholdedline featurestype in the thresholdvalue 45

Above: A threshold value of 45 retains most linefeatures unbroken, and minimizes background noise.Above Left: The threshold value is too high, andnoise from the background intrudes. Left: With a lowthreshold value, the line features lose their integrity.

A locator box in the RasterThresholding and Tracingwindow shows whichportion of the image istreated by the SampleResults window. Drag thebox to a different location tosee how the currentthreshold settings apply toa different part of the rasterimage.

page 6

Digitizing Soil MapsPart One: Digitize One Section

Closing Gaps (End-to-End)If you set the threshold value low enough to sup-press all the background image noise, you will usu-ally have some small gaps in your line features. TheClose Gaps (End-to-End) filter joins gapped line el-ements as they are traced. The control value speci-fies how many cells you want the filter to jump as itcloses gaps. If you set the value too low, your lineelements will not be continuous and you will notget closed, complete soil polygons. If you set thevalue too high, the filter will start joining all thedashed drainage lines and give you lines that youdo not want.

Move the locator box to an area that shows gaps inline features. You may wish to press one of thezoom icon buttons at the top of the Sample Resultswindow to get a closer look. Turn on the CloseGaps (End-to-End) toggle, and click the [Test] pushbutton. The process traces the line features in ma-genta, showing the threshold results with line gaps,and then applies the gap filter, showing the result-ing traces in green (gaps closed).

The filters are processed inan order that is designed tohave a cumulative effect.Thus, you should adjusteach filter in turn and besatisfied with its resultsbefore going on to the nextfilter. If you skip filters and,for example, RemoveDangling Lines withoutapplying the earlier CloseGaps filter, you could seeneeded line elements dropout.

No matter how carefully youset the threshold value, someline features will probablybreak. The Close Gaps (End-to-End) filter jumps gaps of awidth you specify. Set thevalue for how many cells youwant the filter to jump. If youset the filter for too large agap, you will see otherfeatures connected, such asdrainage lines joined to theirdashed line segments.

STEPSturn on the Close Gaps(End-to-End) toggleadjust the location andzoom so you can seeline features with gapsin the Sample Resultswindowclick [Test] to see theresults of the filter

page 7

Digitizing Soil Maps Part One: Digitize One Section

Closing Gaps (End-to-Line)Mid-line gaps, such as those treated with the End-to-End gap filter are common. Occasionally you mayalso see a gap between the end of one line and themiddle of another where they should join at a “T”junction. Use the Close Gaps (End-to-Line) filter tojoin such line features.

When you use a threshold value of 45, theSHEET51SECT30 sample data provided for these exer-cises does not have any end-to-line gaps that needto be closed. Nevertheless, you can see how thefilter works with the crossing drainage lines in theMeC2 soil unit adjacent to the bounding sectionroad at the top of the image. If the two drainagelines that cross the soil unit were supposed to bebisecting line features, you could close the gaps(illustrated below) by increasing the control value.One of the gaps closes at 4.0, and both gaps close at5.0.

Turn on the Close Gaps (End-to-Line) toggle andexperiment with a few different control values. Re-member to set the value back to 3.0 before you goon to the next exercise.

STEPSturn on the Close Gaps(End-to-Line) toggleadjust the location andzoom so you can seethe MeC2 soil unitillustratedchange the controlvalue to 5.0click [Test] to see theresults of the filterchange the controlvalue back to 3.0

You may encounter placeswhere there is a gapbetween two lines thatshould join at a “T” juncture.The Close Gaps (End-to-Line)filter jumps gaps of a widthyou specify. Set the valuefor how many cells you wantthe filter to jump. If you setthe filter for too large a gap,you will see other featuresconnected, such as drainagelines joined to polygonboundaries (as illustrated).

TIP: Apply the End-to-Linegap filter even if you don’thave obvious open “T”junctions. Part of the filter’sjob is to tie up charactergaps in the soil type labels,which makes them easy todelete with the subsequentIsland Polygon filter (seepage 9).

You can also closegaps with the

Undershoots filter in theVector Tools of the SpatialData Editor.

page 8

Digitizing Soil MapsPart One: Digitize One Section

Removing Dangling Lines

Since the goal of theprocess as applied to soilmaps is the extraction ofclosed polygons, any linethat ends without joininganother line can beassumed to be an un-wanted artifact. Crossingdrainage lines, as illus-trated, count as “danglinglines” which, if shorter thanthe specified length, thisfilter will drop during theconversion processing.

Vocabulary: A DanglingLine intersects another lineelement on only one end.The unattached endterminates in a nodeelement that is not sharedby another line element.After unwanted gaps havebeen closed, any remainingdangling lines are probably“nuisance” features (suchas drainage lines or railroadhash marks).

STEPSturn on the RemoveDangling Lines toggleadjust the location andzoom so you can seeline features with shortcrossing lines in theSample Results windowclick [Test] to see theresults of the filter

Crossing lines are common artifacts in soil mapsand almost always need to be removed. Carefulapplication of the Remove Dangling Lines filter cancatch most crossing lines before the output vectorline elements are created.

Turn on the Remove Dangling Lines toggle. Theassociated Length control value determines themaximum length of line features (measured in num-ber of cells) that will be deleted. The longer a line is,the more likely it is part of a feature that you want tokeep. The initial Length value from the settings inSECT30.INI is 111 cells. Experiment with longer andshorter values and inspect the results produced byclicking [Test].

IMPORTANT: If you are processing a section (un-like this sample data ) that is not bounded by linefeatures, then you will have many dangling line fea-tures where lines run off the edge without meeting aboundary line. With such material, this filter may beuseful, only if you turn on the Include Implied Bor-der toggle to create out-side boundary lines.

You can also removedangling lines with

the Dangling Lines filter inthe Spatial Data Editor.

page 9

Digitizing Soil Maps Part One: Digitize One Section

Removing Island Polygons

STEPSturn on the RemoveIsland Polygons toggleadjust the location andzoom so you can seecharacters in a soil typelabelclick [Test] to see theresults of the filterexperiment with otherSize values, butremember to change thevalue back to 144before you go on to thenext exercise

Extracting large polygons isthe goal of the digitizationprocess. But small, islandpolygons are probably labelartifacts and the RemoveIsland Polygons filtereliminates them in theconversion. Set the filtersize large enough to catchlabel artifacts, but not solarge that the filter elimi-nates any legitimate islandsoil polygons.

Small island polygons should usually be treated asartifacts that need to be removed. Especially aftergaps have been joined and dangling lines removed,label features may be little more than clusters of smalladjacent polygons.

Turn on the Remove Island Polygons toggle. Theassociated Size control value determines the maxi-mum size of polygon features (measured in numberof cells) that will be deleted. The larger a polygon is,the more likely it is an actual soil polygon or otherfeature that you want to keep. The initial Size valuefrom the settings in SECT30.INI is 144 cells. Experi-ment with larger and smaller values and inspect theresults produced by clicking [Test].

Remember that an island polygon may be anythingfrom a one-line solitary polygon to a complex of ad-jacent polygons that may themselves contain nestedisland polygons. Set the Size value with some careand inspect the sample results in several parts of theimage.

You can also removeisland polygons with

the Remove Islands filter inthe Spatial Data Editor.

page 10

Digitizing Soil MapsPart One: Digitize One Section

Small “bubble” polygonsare created by the linethinning process whenthick line features haveinterior dropout cells. Setthe Remove BubblePolygons filter to removesuch polygons from lineelements. If you set thefilter value too high, youmight remove wantedpolygon features.

STEPSturn on the RemoveBubble Polygons togglescroll along theboundary roads so youcan see line featureswith bubbles in theSample Results windowclick [Test] to see theresults of the filter

Removing Bubble PolygonsThe Auto-Trace process works on selected line fea-tures by thinning each line until it is one cell wideand then tracing the thinned result. If a thick linefeature has dropout cells inside, then the thinningprocess creates small bubble polygons around theholes in the line. Bubble polygons are a nuisance,not only because of their appearance, but also be-cause they provide misleading polygon statistics inthe resulting vector object: when you look at thepolygons statistics, you see more polygons thanyou expect, and you have a number of polygonsthat have no soil type attributes. The bubble poly-gon filter breaks a bubble polygon by removing oneif its sides.

Turn on the Remove Bubble Polygons toggle. Theassociated Size control value determines the maxi-mum size of polygon features (measured in numberof cells) that will be deleted. The initial Size valuefrom the settings in SECT30.INI is 11 cells. Experimentwith larger and smaller values and click [Test] toinspect the results.

You can also removebubble polygons with

the Remove Bubbles filter inthe Spatial Data Editor.

page 11

Digitizing Soil Maps Part One: Digitize One Section

STEPSturn on the Thin toggleclick [Test] to see theresults of the filterchange the Factor valueand method optionbutton, and inspect thedifference in the resultschange the method andFactor back to MinimumDistance and 3.0click [Run] and specifySECT30TRACE for theoutput vector objectwhen the processfinishes, select Exitfrom the File menu toquit the Auto-Traceprocess

The value for the thinningfactor is too high, and thetrace line cuts cornersinstead of following thecurve smoothly.

A lower thinning factorminimizes the zig-zag effectand lets the trace follow theline feature closely.

Thinning and ProcessingThe Auto-Trace process traces the thinned rasterlines by placing a vertex at every point where thepath of the line changes direction. In most cases theresulting line elements have a denser set of verticesthan is necessary, and can be thinned.

Turn on the Thin toggle. The associated Factor valuedetermines how many vertices will be discarded inthe line traces. The initial Factor value from the set-tings in SECT30.INI is 3 for the Minimum Distancemethod. Experiment with larger and smaller valuesand click [Test] to inspect the results.

The thinning filter is the last control value to set.You can apply the current thresholding and filteringvalues to the input raster object by clicking the [Run]push button. TNTmips opens the standard SelectObject window; name the new vector objectSECT30TRACE. Since the Auto-Trace process convertssome features that you don’t want, the next exer-cises show how to clean the output vector object inthe TNTmips Spatial Data Editor.

You can also thinlines with the Line

Simplification filter in theSpatial Data Editor.

page 12

Digitizing Soil MapsPart One: Digitize One Section

Clean ArtifactsAfter you create a vector object in the Auto-Traceprocess, you need to open it in the Spatial DataEditor to clean and repair its elements. Some lineelements need to be deleted and some need to bereshaped.

Open the Spatial Data Editor (Edit / Spatial Data).Add the SHEET51SECT30 raster object as a referencelayer. Open SECT30TRACE for editing, and change theline and node colors and thickness to somethingthat will be easy to see over the reference layer (theillustrations on the following pages use red lines,two pixels wide with white nodes).

It is best to traverse the vector object systemati-cally, editing as you go. For these exercises, start in

the top left corner zoom in,and scroll the view back andforth across the objects.

If any of the editingtechniques mentioned hereare unfamiliar, review thetutorial Editing VectorGeodata.

A particularly tangled intersection of railroads, labels, drainage annotations, and soilpolygons comes out of the Auto-Trace process with several artifacts that need to beremoved. Watch for the incorrect boundaries between polygons that are sometimescreated along the line where a drainage or road passes through a polygon.

STEPSselect Spatial Data fromthe Edit menuadd the SHEET51SECT30raster object as areference layeropen SECT30TRACE foreditingchange the line style tored, 2 pixels wide.move to the top leftcorner and zoom in

The vector filters in the SpatialData Editor can be used to catchany artifacts that you missed inthe Auto-Trace process.

page 13

Digitizing Soil Maps Part One: Digitize One Section

Remove LinesTraverse the vector object in the Spatial View win-dow, moving the view so youcan edit each part in turn. Yourprimary task is to identify anddelete line elements that are notpart of soil polygon boundaries.In a typical conversion, some drainage lines, roads,label artifacts, and railroad features must be selectedand deleted.

Select the SECT30TRACE layer and click the Edit Ele-ments icon button in the Vector Tools window. Inthe Spatial Data Editor window, turn off the selec-tion icon for each element type except lines and clickthe Select icon button. In the Element Selectionwindow, select the Delete Operation icon. Then goto the Spatial View window and select several un-wanted line elements. Click the [Selected] push but-ton to delete them.

STEPSclick Edit Elementsin the Vector Toolswindowselect the Mode,Element Selection,and Operationtools as illustratedclick on severalunwanted lineelements in the SpatialView windowclick [Selected] to deletethem

Turn offselection forall elementtypes exceptlines.

Line elements outside the southwest corner ofsection 30 are selected and ready to delete.

Add severalunwanted lineelements to theselected set with themultiple select mode.

Select the Delete operation and click[Selected] to apply the Delete operation toall the selected line elements at once.

Use the editingcontrols in theSpatial DataEditor andElementSelectionwindows.

page 14

Digitizing Soil Maps

As you delete line elements, excess nodes remain at formerintersections. Click Remove Excess Nodes in the Vector Toolspalette. Right: after excess nodes have been removed, youcan edit zig-zag lines.

several lineartifacts aredeleted

remaining lineshave excessnodes

excess nodesremoved

Part One: Digitize One Section

Remove Excess Nodes

STEPSclick RemoveExcess Nodes inthe Vector Tools paletteselect Save from theFile menu

Vocabulary: An “excessnode” is a node elementshared by exactly two lineelements of the same type.For some applications, youmight want to keep the twolines divided, if for example,the lines were to beassigned differentattributes. For soil mapdigitization, remove theexcess node to create oneline element from the two.

The topology of a vector object specifies that anode element always marks the end of each lineelement. Thus there is always a node element atevery intersection of line elements. When you de-lete an intersecting line element, you leave behindan “excess node” in the remaining line element atthe former intersections.

To update the object topology and remove the ex-cess nodes, click the Remove Excess Nodes buttonin the filters panel of the Vector Tools palette. Thenselect Save from the File menu to update yourchanges. (It is good practice to save your editseach time you remove excess nodes or scroll to anew part of the vector object.)

After the excess nodes have been removed, youcan drag, delete, and insert vertices to correct theshape of the polygon boundaries, especially whereformer intersections have a zig-zag appearance.

page 15

Digitizing Soil Maps Part One: Digitize One Section

Other Editing Operations

Take A Final Look.This is the last exercise inthis booklet that treats thesimple one-sectiondigitization project. Whenyou finish editing, youshould have a completevector object with clean soilpolygon elements thatmatch those on the inputraster object. Before yousave your final result andexit the Spatial Data Editor,take an overall look at thevector object to catch anymissing or incorrectelements.

STEPSinspect the vectorobject for remainingproblemsapply appropriateediting toolssave the vector objectand exit the Spatial DataEditor

The jagged boundary line elements around the boundaries of the map can beselected and straightened with the straighten tool.

To finish this digitization exercise, you will use theSpatial Data Editor to make final corrections to theSECT30TRACE vector object. You have removed un-wanted line elements; now check to be sure that norequired line elements are missing.

Use the editing tools on line elements to in-sert, delete, and move vertices to correct places

where traced line elements do not conform to soilboundaries in the reference layer.

Use the straighten tool on line elements thatshould be straight, such as the roads along

the boundaries of the section. The straighten toolremoves all vertices between the end nodes.

You might use the spline tool to enforce acurved appearance on line elements that have

especially visible corners. However, the spline op-eration adds many new vertices to a line element.Weigh the benefits of what may be a small cosmeticgain against the increased storage and processingdemands of denser line elements.

This concludes the basic digitization exercise forPart One of this booklet. You probably will want toenhance the soil map by attaching databases andassigning styles. These steps are treated in PartTwo of this booklet and in other tutorials in thisseries.

page 16

Digitizing Soil MapsPart Two: For A Larger Project

Scanning Soil Sheets

STEPSscan your soil sheetswith Process / Raster /Utilities / Capture TWAINor with an externalscanning utilitytry a few test scans tofind the best resolution,contrast, and brightnessif you used an externalscanning utility, importthe rasters withProcess / Import-Export(Refer to the tutorialImporting Geodata)

Sheets 45 and 51 from theLancaster County SoilSurvey have been scannedand georeferenced.

This is the first exercise in Part Two of this booklet.From here on, the exercises are not strictly sequencedand they treat general issues to consider if you plana larger digitization project.

If you plan a large digitization project, such as cre-ating one digital soil map of an entire county, beginwith good scans of all the soil sheets involved.

Choose a high enough scanning resolution so thatthe line features are several cells wide. For example,the soil map used in Part One (pages 3-15) wasscanned at 300 dpi and then sampled to an effective150 dpi, which provided a usable map. A 600 dpiscan would be no better: tracing, thinning, and pro-cessing times would increase, not to mention theincreased size and disk storage requirements of theraster objects. Do some test scans at different reso-lutions and compare the results.

Scan in grayscale mode, notcolor mode. Adjust your scan-ning software controls for con-trast and brightness. Choosesettings that give solid, blackline features that are distinctfrom the gray photo image inthe background.

Position the soil sheets on thescanner carefully to minimizethe amount of angular skew-ing.

page 17

Digitizing Soil Maps Part Two: For A Larger Project

Adding Georeference

Since the soil maps use anuncorrected airphoto base,you can expect largerresiduals than you wouldsee with orthogonal mapmaterials. Enter the pointsas accurately as you can,and let your subsequentwarping or rectificationsteps reconcile the internalgeometry.

Add georeference points on each of theprinted map crosshairs. If you get manylarge residual values, you may havemisplaced a control point.

STEPSadd georeferencecontrol to each soilsheet with Edit /Georeference

Georeference is the key to creating a mosaic of allthe soil sheets in your project. Since soil surveysare created and published on uncorrected airphotos,the non-orthogonal geometry of the soil sheets pre-vents them from being simply tiled together. Goodgeoreference control lets the mosaic process warpall the soil sheets to a common reference grid, ensur-ing a good fit.

Use the TNTmips georeference process (Edit /Georeference). Refer to the TNT Reference Manualand the tutorial booklet Georeferencing for review ifyou need it. The soil sheets are printed with map tickmarks, so put your control points on those marks.You can also enter control points for road intersec-tions or other features if you know their map coordi-nates from surveys, GPS readings, or other map ma-terials.

Pay some attention to the Residual values. Sincethe soil sheets are based on uncorrected airphotos,large residuals for some points can be expected, butconsistently large residuals for many points may in-dicate an incorrectly placed control point.

page 18

Digitizing Soil MapsPart Two: For A Larger Project

Poor georeference controland uncorrected airphotogeometry can causeproblems along the seamsin the mosaic process.The illustration shows howtwo sheets join along a linethat does not overlap itselfand creates unwantedseam artifacts. If you getsuch complex line results,correct your georeferencecontrol and consider usinga different warping methodin the mosaic process.

If you have an elevationraster object that includesthe extents of the soilmaps, you can produce“orthoimage” soil sheetswith improved geometrybefore coming to theMosaic process. (Process/ Raster / PhotogrammetricModeling.) Refer to the TNTReference Manual and tothe tutorial Making DEMsand Orthophotos.

Mosaic Before Auto-TraceThe TNTmips Mosaic process (Process / Raster /Mosaic) can use georeference control to assembleany number of raster objects into one. As the pro-cess makes the mosaic, it automatically adjusts theorientation and scale of each object.

It is possible to autotrace each separate soil sheetand then combine the resulting vector objects.However, it is slightly harder to reconcile line ele-ments at the map seams when merging multiple vec-tor objects than to edit the seam artifacts in singlevector object that was auto-traced from a mosaickedraster object. When a soil polygon straddles a seam,its line elements need to meet end-to-end. You maysee overlapping, mismatched, and double bound-ary lines when you merge vector objects. By con-trast, a raster mosaic usually yields a single bound-ary line, and it is easier to handle lines at the seam asyou edit the auto-traced vector object.

The Mosaic process automatically warps each com-ponent raster object before it combines them. Ifyou get seaming problems in the result (as illus-trated below) choose a different warping method orcorrect your georeference control.

page 19

Digitizing Soil Maps Part Two: For A Larger Project

The Mosaic process inTNTmips offers manypowerful features forcreating good mosaics —even from images that havepoorly matched color,contrast, brightness, andother conflicting character-istics. Refer to the TNTReference Manual and tothe tutorial MosaickingRaster Geodata.

Set the overlap method to “minimum” so that the whitemargin around each soil sheet (high cell values) willbe discarded while the lower image cell valuessurvive. Other seaming methods are appropriate forother kinds of mosaic materials.

The “minimum”method discards thehigh cell values ofthe white margins inthe overlap areawhile using darkerimage and label cellvalues. Notice thatthe “Joins sheet”notations from themargins of adjacentsheets are present inthe mosaic result.

Mosaic SeamsThe Mosaic process has several ways of treatingthe overlap area between adjacent raster objects.Since each soil sheet has a white margin that con-flicts with the image area in adjacent sheets, youneed a seaming method that always chooses imagecells over margin cells in overlap areas.

The illustration below shows how four seaming meth-ods treat white margins in the overlap area. In theLast Raster method, the margin of the sheet on topblots out the image in its neighbor. In the Maximummethod, margin cells from both sheets are selected.In the Average method, margin values are averagedwith image values. In the Minimum method, thedarker image cells are always preferred over the whitemargin values.

averagelast raster maximum

minimum

page 20

Digitizing Soil MapsPart Two: For A Larger Project

Import Attribute Tables (MUIR)

You can order national dataon CD-ROM from:USDA - Natural Re-

sources ConservationService

National Cartography andGeospatial Center

501 Felix Street, Building23 (P.O. Mail 6567)

Fort Worth, TX 76115Telephone: (800)672-5559

The soil attribute tables that are printed in the countysoil survey books are available in electronic form asthe Map Unit Interpretation Record (MUIR) data-base. You can buy a complete MUIR on CD fromUSDA Natural Resources Conservation Service, ordownload individual tables from the Web. If youuse the Web page for download, check the boxesfor every type of table, and check the Yes box toinclude the MUIR header rows (which are used bythe TNT import process).

Use the TNTmips import process (Process / Import-Export) to import the tables to your vector objects(polygon elements).

Find the MUIR Web page athttp://ortho.ftw.nrcs.usda.gov/cgi-bin/muir/dmuir.cgi

or use a Web search facility (such as www.google.com) withthe search phrase “Download MUIR”

Click Yes to include theMUIR header rows.

page 21

Digitizing Soil Maps Part Two: For A Larger Project

Attach Attributes

Select a primary table thathas fields common to manytables. After you havelinked each polygon to itssoil type, in the primarytable, you can quickly relateseveral other tables to thesoil polygons by relatingother tables to fields in theprimary table. Thereafter,database information fromall the related tables can beaccessed from the polygonelements.

To attach database records to polygon elements, atleast one of your database tables must have a keyfield defined. Select Edit / Attribute Databases, se-lect the polygon elements from the vector soil mapobject, and open one of the MUIR tables. SelectTable / Edit Definition, choose a field, and click itsPrimary Key toggle. (Review this procedure in moredetail in the TNT Reference Manual and in the tuto-rial Editing Vector Geodata.) You can use the toolsin the Polydata window to establish relations to othertables. (Refer to the tutorial Managing RelationalDatabases.)

After a primary key has been defined, you can use aselection query in the display process (Display /Spatial Data) to visit each polygon element in turnand attach its primary database record. Add the soilsheet mosaic of raster objects for background refer-ence, and overlay your editedvector object. For a key fieldnamed “Class,” the selectionquery would have the form

SetNum(Class[*])<1Enter your polygon query, andclick the Next icon button.This query pans to the nextpolygon that does not have anattached attribute in the Classfield. Zoom in on the polygonelement and read the soil typefrom the soil sheet referenceobject. Assign the soil type at-tribute from the table and clickthe Next icon button to ad-vance to the next polygon ele-ment that needs an attribute.

After each polygon hasbeen related to a databasetable, you can assigndisplay styles according tothe soil type associatedwith each polygon.

page 22

Digitizing Soil MapsPart Two: For A Larger Project

The illustrations andexamples in this booklet usesimple black borders andsolid fills for polygon styles.Refer to the TNT ReferenceManual and the tutorialCreating and Using Stylesfor details on creating andusing fill patterns andtransparency effects.

TIP: Choose similar colorsfor all the soil varieties in aseries. For example, in thesample data, all the types inthe Mayberry series havepolygon fill colors selectedin the red and maroon hues.

Create and Assign StylesAfter you have attached database tables to poly-gon elements through a key field, you can createdrawing and display styles for each attribute value.Open the Vector Object Display Controls window inthe display process, and click the Polygons tab. Setthe Style option button to By Attribute and click[Specify] to open the Vector Object Key AttributeStyle Assignment window (illustrated below). Theleft half of the window lists the key field values fromthe database table. The right panel lists availablestyles. (To begin with, the Available Styles list con-tains a single item: <use default style>.)

Click [Edit] to open the Style Editor window for eachattribute value, click the New Style icon button, typein a Name, and select Border and Fill styles. (Referto the TNT Reference Manual for more detail on theStyle Editor.) When you have a style for each at-tribute type, click [OK] to close the Style Editor win-

dow. The Available Styles list now showsall your new styles. Select each attribute inturn from the list on the left, click on itsstyle in the styles list, and click [Assign].

For each style:

1. click New Style

2. type a Name

3. pick Border andFill styles

After creating allstyles, click [OK]to return

Select an attribute, then astyle, then click [Assign]

page 23

Digitizing Soil Maps Part Two: For A Larger Project

Keep up with the latestversion of TNTmips.Processes such as thoseused in this booklet areupdated in regularupgrades. Download thelatest version of TNTlitefrom the MicroImages Website:www.microimages.com.

The vector object digitizedfrom SHEET51SECT30 isdisplayed on soil sheet 51.The Hardcopy Layoutprocess in the TNT displayprocess trimmed the soilsheet to chosen extents,added a map grid, andadded the map title lines.Vector polygon labels weregenerated automaticallyfrom attached attributetables.

Soil Map ApplicationsThe soils layer is one of the most important in anyadvanced system for spatial data analysis. Soil at-tributes are important for engineering, construction,planning, agriculture, natural resource management... almost every discipline that uses geospatial infor-mation needs to have access to soils data. Querysoil type attributes with other GIS layers that con-tain data for crop yield, proximity to developmentproposals, erosion data, watershed analysis, engi-neering studies, and any number of other spatialmeasurements and conditions.

The final use you make of your digitized soil mapdepends on the needs of your organization. Youcan use the soil polygons and their attributes inmany complex GIS analyses in the TNT products.You can create map displays and large-format prints.You can export the vectorobject to a wide assortmentof popular formats (includ-ing AutoCAD, Arc/Info, andMapInfo) for use in othersystems.

page 24

Digitizing Soil MapsAdvanced Software for Geospatial AnalysisSOIL

MAPPING

Voice: (402)477-9554www.microimages.com

MicroImages, Inc. publishes a complete line of professional software for advanced geospatial datavisualization, analysis, and publishing. Contact us or visit our web site for detailed productinformation.

TNTmips TNTmips is a professional system for fully integrated GIS, image analysis,CAD, TIN, desktop cartography, and geospatial database management.

TNTedit TNTedit provides interactive tools to create, georeference, and edit vector, image,CAD, TIN, and relational database project materials in a wide variety of formats.

TNTview TNTview has the same powerful display features as TNTmips and is perfect forthose who do not need the technical processing and preparation features of TNTmips.

TNTatlas TNTatlas lets you publish and distribute your spatial project materials on CD-ROM at low cost. TNTatlas CDs can be used on any popular computing platform.

TNTserver TNTserver lets you publish TNTatlases on the Internet or on your intranet.Navigate through geodata atlases with your web browser and the TNTclient Java applet.

TNTlite TNTlite is a free version of TNTmips for students and professionals with smallprojects. You can download TNTlite from MicroImages’ web site, or you can orderTNTlite on CD-ROM.

adjust threshold ......................................... 5attribute tables .................................. 20, 21auto-trace .............................................. 3, 4bubble polygons ..................................... 1 0close gaps ............................................. 6, 7conversion filters ...................................... 3dangling lines ............................................ 8drainage lines ............................................ 3end-to-end ................................................. 6end-to-line ................................................. 7excess nodes ............................................ 14georeference ........................................... 1 7implied border .......................................... 8include implied border ............................ 8intersections ............................................. 14island polygons ......................................... 9label artifacts ............................................. 9labels .......................................................... 3line simplification filter ......................... 1 1mosaic seams, overlap ........................... 19mosaic ...................................................... 18MUIR ....................................................... 2 0node elements ......................................... 14

orthoimage soil sheets ............................ 1 8polygon styles ......................................... 22Raster Thresholding window .................. 4remove bubble polygons ...................... 1 0remove dangling lines ............................. 8remove excess nodes .............................. 14remove island polygons .......................... 9remove lines ............................................ 13sample data ................................................ 2Sample Results window ...................... 4, 5scanning soil sheets ................................ 16soil attributes .................................... 20, 21soil labels ................................................... 9Spatial Data Editor ........................... 12-15spline tool ................................................ 15straighten tool ......................................... 15styles ......................................................... 22thinning .............................................. 3, 11threshold value ......................................... 5tracing ........................................................ 3undershoots filter ...................................... 7vector tools ................................................ 7

MicroImages, Inc.¤

Index