analyzing terrain and surfaces

Analyzing Terrain and Surfaces

page 1

Tutorial

Analyzing Terrainand Surfaces

with

TNTmips®

TERRAIN

ANALYSIS


page 2

Before Getting StartedTopography profoundly influences many physical and biological processes andprovides the backdrop for human activities such as construction, transportation,communication, resource management, and recreation. Because of the variedways in which natural or manmade systems interact with landscapes, computeranalysis and modeling of terrain requires a number of specialized software tools.This booklet introduces a series of TNTmips® processes that allow you to ana-lyze elevation rasters and to model various types of interaction with terrain.

Prerequisite Skills This booklet assumes that you have completed the exercisesin the tutorials entitled Displaying Geospatial Data and Navigating. Those exer-cises introduce essential skills and basic techniques that are not covered againhere. Please consult those booklets for any review you need.

Sample Data The exercises presented in this booklet use sample data that isdistributed with the TNT products. If you do not have access to a TNT productsCD, you can download the data from MicroImages’ web site. In particular, thisbooklet uses sample files in the TERRAIN data collection. Be sure the sample datahas been installed on your hard drive so changes can be saved as you use theseobjects in the following exercises.

More Documentation This booklet is intended only as an introduction to terrainand surface analysis. Details of the process can be found in a variety of tutorialbooklets, color plates, and Quick Guides, which are all available fromMicroImages’ web site (go to http://www.microimages.com/search to quicklysearch all available materials, or you can narrow your search to include onlytutorials or plates.

TNTmips and TNTlite® TNTmips comes in two versions: the professional ver-sion and the free TNTlite version. This booklet refers to both versions as“TNTmips.” If you did not purchase the professional version (which requires asoftware license key), TNTmips operates in TNTlite mode, which limits the sizeof your objects and does not allow preparation of linked atlases. All the exer-cises can be completed in TNTlite using the sample geodata provided.

Randall B. Smith, Ph.D., 27 September 2007©MicroImages, Inc., 2001-2007

It may be difficult to identify the important points in some illustrations without acolor 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 newest tutorialbooklets on other topics. You can download an installation guide, sample data,and the latest version of TNTlite.

http://www.microimages.com


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STEPSX choose Main / Display

from the TNTmips menu

Welcome to Analyzing TerrainTNTmips provides a number of tools for visualizingand analyzing Digital Elevation Models (DEMs).Appropriate contrast enhancement and use of colorpalettes can significantly aid in visualization ofDEMs in a 2D display. A DEM can also be dis-played with relief shading, which helps you visualizethe surface by portraying it as if it were illuminatedfrom a particular compass direction and elevationangle, both of which you can adjust interactively.These tools are also applicable to other rasters thatrepresent 3D mathematical surfaces, such as griddedgravity or crop yield values.

The Topographic Properties process computes gen-eral terrain characteristics from a DEM: slope, aspect,plan and profile curvature, and shading. Slope andaspect refer to the magnitude and direction, respec-tively, of maximum downward slope. Slope, aspect,and curvature rasters can be used as components inmore complex environmental models, such as pre-dicting soil erosion or landslide hazards. The shadingraster provides a fixed alternative to displaying theDEM with interactive relief shading.

The Viewshed process performs line-of-sight analy-sis of a DEM to define a viewshed, the portion of theterrain that is visible from a given viewpoint on orabove the ground. Viewshed analysis can be used tofind optimal sites for communication facilities suchas television or cell phone transmitters or for mili-tary observation posts or fire towers. It can also beused to assess the visual impact of activities such asmining and logging.

The Cut and Fill Analysis process compares two el-evation rasters of the same area and identifieslocations where their elevation values differ. Theseareas are traced to form polygons in an output vec-tor object. The volume of material added orsubtracted is calculated for each polygon and storedin an attached database table.

A companion tutorial bookletentitled Modeling WatershedGeomorphology, introducesthe Watershed process,which computes streamnetworks, watersheds, andrelated properties from aDEM.

Techniques for creatingconsistent and effectivedisplays of DEMs areintroduced on pages 4-8.Pages 9-13 cover theproducts you can create inthe Topographic Propertiesprocess. The Viewshedprocess is discussed onpages 14-18, followed by anintroduction to the Cut andFill process on page 19.


page 4

STEPSX press the Add

Raster icon buttonin the Display Managerwindow and chooseSingle from thedropdown menu

X navigate to the MATCH

Project File in the TERRAIN

data collection andselect rasters EAST andWEST

X right-click on the rastericon for the WEST layer inthe Display Manager andselect Enhance Contrastfrom the dropdownmenu

X in the Raster ContrastEnhancement window,change the value in theleft (minimum) InputRange box from 1340 to1280

When you work with a set of adjacent DEM or othersurface rasters, each raster will have a different rangeof values, but the same numerical value has the samemeaning in each. To convey that meaning consis-tently when the rasters are displayed, a given rangeof surface values should be displayed with the samerange of gray tones (or colors) in each raster. Achiev-ing that consistency requires that you adjust thecontrast enhancement for each raster.

The problem is illustrated by thetwo DEMs used in this exercise.Elevations in raster EAST rangefrom 1280 to 1707 meters andin raster WEST from 1340 to 2741meters. The default linear con-trast table that has been savedwith each raster stretches the fullrange of gray tones from eachraster’s minimum to its maxi-mum value. As a result, the samegray tones correspond to differ-

ent elevation ranges in each raster and the DEMs donot appear to match along their common boundary.

To properly adjust the contrast, you should first ex-amine the histograms of all the rasters in the set todetermine the overall minimum and maximum val-ues. For rasters EAST and WEST the overall range isfrom 1280 to 2741. You can then open the RasterContrast Enhancement window for each raster andset the Input Range values to match the overall rangeof the raster set rather than the raster’s own particu-lar range. (Alternatively, use the Project FileMaintenance process to copy the first adjusted con-trast subobject to all subsequent rasters.) Gray tonesare then spread over this larger overall range for eachraster, producing consistent gray tones for the cor-responding elevation ranges in each (see illustrationon the following page).

X choose Save from theEnhancement window’sFile menu, then chooseClose This exercise continues on

the following page.

Set Consistent Contrast and Colors I


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STEPSX repeat the last three

steps for the EAST layer,but change the right(maximum) Input Rangevalue from 1707 to 2741

X redraw the Viewwindow

X right-click on the rastericon for the WEST layerand select Edit Colorsfrom the dropdownmenu

X click on the Palettemenu in the ColorPalette Editor windowand select the EarthTones palette

X if the Earth Tones paletteis not shown on theinitial menu, chooseMore Palettes and selectit from the scrolling list inthe Standard ColorPalettes window andclick [OK]

X choose Save As fromthe Color PaletteEditor’s File menu andsave the palette as asubobject of raster WEST

X repeat the last step andsave the palette as asubobject of raster EAST

Rasters WEST and EAST displayedwith gray tones in each rasterspread linearly over the overallelevation range. Gray tones nowmatch at the boundary.

Set Consistent Contrast and Colors IIColor is usually more effective than gray tones inbringing out detail in a displayed DEM or surfaceraster. Once you have set up a consistent contrasttable for each raster, you can use the Color PaletteEditor to select a standard color palette or to designyour own. (A linear contrast enhancement is rec-ommended if you are going touse a color palette.) The paletteshould be saved as a subobjectfor each raster in the set. Thesame color is then assigned tothe corresponding elevationrange in each displayed raster.

EarthTones palette applied to rasters EAST and WEST.

The Earth-Tones colorpalette, one ofmany StandardColor Palettesavailable inTNTmips.

When you have completed this exercise, right-click onDisplay Group 1 in the Display Manager and selectClose Group from the dropdown menu.


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Display DEM with Relief ShadingSTEPSX press the Add

Raster iconbutton in the DisplayManager and chooseSingle

X select raster CLKDEM fromthe SHADE Project File

X right-click on the rastericon for the CLKDEM layerand select ReliefShading from thedropdown menu

X vary the Azimuth settingin the Relief ShadingAdjustment window, click[Apply] and note theeffect on the DEM

X vary the Elevation settingand note the effect

X vary the Z Scaling settingand note the effect

DEM of CraterLake area,Oregon ingrayscale.

Crater Lake DEMdisplayed withrelief shadingsettings shown inthe illustrationabove.

Close the Relief Shading Adjustment windowand close Display Group 1 when you havecompleted this exercise.

NOTE: you can change the display parameters for araster so that it is automatically displayed with reliefshading each time you add it to a group. To do so,open the Raster Layer Controls window and turn onthe Relief Shading toggle button. The last-used (ordefault) shading settings will be used. Any changesin the shading settings that you make subsequentlyfor the raster using the Relief Shading Adjustmentwindow are not automatically saved. To force asave of the new settings, turn the Relief Shadingtoggle button off and then back on.

The Relief Shading tool shows how the surfacewould appear if illuminated by an infinitely distantlight source (assuming that the surface represents auniform material). The Relief Shading Adjustmentwindow allows you to vary the azimuth (compassdirection) and elevation angle of the light source andthe Z-scaling (vertical exaggeration). The azimuthcan vary from 0 to 360 degrees clockwise from north.Surface features perpendicular to the illuminationdirection are accentuated by shadowing, while thosetrending parallel to it are less visible. Decreasingthe elevation angle generally darkens the shadedimage and increases the contrast between shadowedand illuminated areas. To produce a brighter image

that preserves shadow contrast, increase boththe elevation angle and the z-scaling.


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Create a Color Shaded Relief DisplaySTEPSX in the Display

Manager, addraster object MWDEM1from the SHADE ProjectFile

X note the relief-shadedview of the DEM

X add raster objectMWDEM2 from theSHADE Project File

X note the color-mappedview of the DEM

X right click on the rastericon for the MWDEM2 layerand select Controls fromthe dropdown menu

X on the Options panel ofthe Raster LayerControls window, changethe value in theTransparency field to 55,then press [OK]

You can combine the effects of color-mapped eleva-tion and relief shading to create color shaded reliefdisplays. If you expect to use such a display repeat-edly, it is best to work with two copies of the DEM,which can be set up to use different display param-eters. Set up one copy of the DEM to display withrelief shading, and the other to display with a colorpalette. Then display both DEMs in the same dis-play group, with the color-mapped version on toppartially transparent. The resulting view combinesthe textural information from the shaded layer withthe color-coded elevation information from the over-lying layer.

You can vary the brightness and contrast of themerged view by adjusting the relief shading settingsfor the lower DEM; a relatively bright shaded im-age produces brighter colors. Vary the transparencysetting of the color-mapped DEM to control the rela-tive contribution of the color and shaded versions.Increasing the transparency will subdue the colorsand place more emphasis on the terrain shading.

Relief-shaded view ofMWDEM1.

Color-mapped view ofWMDEM2.

Color shaded relief view ofthe two versions of the DEM.

Close the display group when you have completed this exercise.


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Display Terrain in 3DSTEPSX choose Display / Open

on the Display ManagerX select DISPLAY GROUP3D

from the SHADE ProjectFile

Add Terrain icon button

Close the display group when you have completed this exercise.

To enhance your visualization of the topography de-picted by a DEM, you can create a 3D perspectiverendering of the DEM in the Display process. Inthis exercise you open a saved display group thatincludes a perspective view of the data you workedwith in the previous exercise (shown below). In thisgroup one of the copies of the DEM raster has beenadded as a Terrain layer to provide a 3D surface uponwhich other layers can be draped. In this examplethe two DEM copies have been used as drape layersto again depict color shaded relief. Controls on theperspective view window allow you vary the 3Dviewing geometry (heading, pitch, and distance).More details on 3D viewing can be found in the tu-torial entitled 3D Perspective Visualizaton.

MWDEM1 as aterrain layer


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STEPSX choose Raster /

Elevation / TopographicProperties from theTNTmips menu

X click [Raster...]X navigate to the SLOPE

Project File and selectobject DEM_S1

X from the Surface-Fittingmenu choose Exact fitto 4 nearest neighborsand center cell

Compute Topographic PropertiesThe Topographic Properties process can be used tocompute several fundamental topographic propertiesfrom the input DEM. You can measure the surfaceslope magnitude (slope) and its downward direction(aspect) at each cell location, and compute measuresof terrain curvature for each cell. Together theseproperties define the spatially-varying shape andorientation of the terrain surface. You can alsocompute shaded-relief images from the DEM withvarying illumination parameters. You can computeany combination of these products in a single run.

A grayscale representation of DEM_S1 withnormalized contrast enhancement. Toaccomodate the range of possible Earth surfaceelevations (in either meters or feet) withoutscaling, many DEM rasters use a signed 16-bitinteger data range (-32,768 to +32,767). Topreserve greater elevation precision, some DEMsare produced in decimal (floating point) meters.Elevations in all of the DEMs used in this bookletare in integer meters.

Choose the surface-fittingmethod to use for computingthe topographic parametersfor each cell.

Use toggles to indicatethe topographicproperties to compute.

Use menus to set theraster type for eachselected output rasterobject.

Horizontal and VerticalCell Size are read fromthe input raster andcannot be edited; alloutput rasters match thecell size and extents ofthe input DEM. Keep the current settings and continue to the next page.

Units menusfor Slope andCurvature


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Compute Slope and Aspect

X press [Run...]X use the standard Select

Objects dialog window toname a new Project Fileand accept the defaultnames for the outputSlope and Aspect rasterobjects

X use the Display processto view theoutput Slopeand Aspectrasterobjects

X close theDisplaygroup whenyou havecompletedthe exercise

STEPSX turn on the Slope and

Aspect toggle buttonsand make sure that theShading and Curvaturetoggles are off.

X choose Degrees fromthe units menu for Slope

X choose 32-bit floatingpoint from the rasterdata type menu for theSlope raster, and 16-bitsigned integer for theAspect raster

Slope raster with auto-normalized contrast.

Slope can be expressed as either a vertical anglemeasured from the horizontal in degrees (0 to 90) oras percent slope [tangent(slope) x 100; a slope angleof 45 degrees is equal to a 100 percent slope].Choose either Degrees or Percents from the optionmenu to make this selection. For maximum preci-sion for the slope values, choose 32-bit floating-pointfor the raster data type. If you wish to quantize slopeangles or percent-ages to the nearestinteger, you can se-

lect 8-bit unsignedfor the raster datatype.

Aspect values are azimuthangles with the range 0 to 360degrees, increasing clockwisefrom north. Flat areas areindicated by a value of -1. In agrayscale display, flat areas andnorth to northeast-facing slopesare darkest and northwest-facing slopes are brightest (theDEM is assumed to be orientedwith north at the top). You canchoose from either 16-bitsigned integer or 32-bit floatingpoint raster data types for theaspect raster.

Aspect rasterwith auto-normalizedcontrastenhancement.


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Compute ShadingSTEPSX turn off the Curvature

toggle and turn on theShading toggle

X set the value of theElevation angle of thesun field to 60

X set the value of theDirection of the sun fieldto 300

X press [Run...]X accept the default

names for the outputshading raster object

The Shading option in the Topographic Propertiesprocess computes and saves a shaded-relief imageof the input DEM. You can use this pre-computedshading image in place of a dynamic shaded-reliefdisplay of the original elevation raster in variousdisplay applications (see pages 6-7). Controls in theParameters panel of the Topographic Properties win-dow let you vary the elevation angle and direction

of the sun illumination aswell as the elevation scale.The shading image is auto-matically displayed in aShading Result window thatopens when the shadingprocessing is complete.

The Method menu providestwo options for computingshading: High-Contrast andDisplay. The High-Con-trast method provides awider range of output shad-ing values than the Display

method (which is identical to the one used to com-pute shading in the Display process). The appearanceof the shading raster will also vary depending on the

contrast enhancement method you use in display-ing it.

The Sun Angle Calculator computes and sets sundirection and elevation angles for the ground po-sition (latitude/longitude), date, and time that youenter. (Time is specified as Coordinated Uni-versal Time, or UTC, also referred to as Zulutime or Greenwich Mean Time.) Use the suncalculator to compute a shading raster to matchthe illumination characteristics of a remote sens-ing image acquired at a known date at time.

Shading raster with auto-linearcontrast enhancement.


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Surface-Fitting MethodsSTEPSX open the Surface-fitting

method menu on theTopographic Propertieswindow

Topographic properties are computed for each cellby using a moving 3 by 3 kernel of cells to computefirst and second derivatives of the local surface inthe line and column directions. The choice of cellswithin this kernel and the weighting factors appliedcan be varied to represent different mathematical ap-proximations of the local surface. The fivesurface-fitting methods shown to the left are avail-able.

The first listed method uses a cross-shaped kerneland fits the surface exactly to all five cell values;this method produces topographic parameters thatare most faithful to the raw elevation values. How-ever, most DEMs contain elevation errors to varyingdegrees. To mitigate the effects of such elevation“noise”, the other four surface-fitting methods useelevations from all nine kernel cells to com-pute a curved surface that is a “best fit”approximation of the kernel values. As a re-sult, these quadratic methods all introduce adegree of averaging and smoothing to the to-pographic parameters. The quadratic methodsdiffer from each other in how the values ofthe more distant corner cells in the kernel areweighted relative to the middle cells along thekernel edges. In addition, only the last qua-dratic method in the list forces the quadraticsurface to exactly match the elevation of thecentral cell in the kernel. Differences betweenthe topographic parameters created using thefour quadratic surface-fitting methods areslight, but may be locally significant.

Exact fit to 4 nearestneighbors and center cell

Quadratic surface, least-squares fit

Quadratic surface, least-squares fit, weighted by1/distance2

Quadratic surface, least-squares fit, weighted by1/distance

Quadratic surface, leastsquares fit, match centralcell

Surface-fitting methods

Exact fit to 4 nearest neighbors

Quadratic surface,least-squares fit

Comparison of local areas of two shading rasterscomputed using different surface-fitting methods.The shading raster from the quadratic surface-fitmethod (bottom) is noticeably smoother than theone computed by exact fit to the 4 nearestneighbors (top).

X press [Exit] on the Topographic Properties window


page 14

Viewshed AnalysisSTEPSX choose Raster /

Elevation / Viewshedfrom the TNTmips menu

X in the Select Objectwindow, navigate to theVIEWSHED Project File andselect object DEM_V1

X move the mouse overthe center of the circlegraphic in the View; thecursor should assumethe four-point arrowshape

X drag the center of thecircle graphic to thelocation shown in theillustration

X with the cursor over theView, use the arrow keyson your keyboard to

The Viewshed process allows you to compute aviewshed from one or more positions on or abovethe surface represented by the input elevation raster.This raster is displayed in a View window and aninitial viewpoint location is automatically placed onthe surface at the center of the raster, indicated bythe number 1 next to the cross at the center of thecircle graphic. You can reposition this point any-where within the area of the elevation raster.

The Test option allows you to preview the results bycomputing a temporary viewshed raster that is dis-played in the Viewshed Analysis window. Thetemporary viewshed raster is displayed with cellswithin the viewshed (visible areas) in white and theremaining cells (non-visible areas) transparent so thatthe input raster is not obscured.

move the viewpoint untilthe Line field in theViewshed Analysis windowreads 391 and the Columnfield reads 158

X click the Test button at thebottom of the ViewshedAnalysis window

The results of a Viewshedanalysis are encoded as abinary raster in which cells witha value of 1 (white) are visiblefrom the selected viewpoints.


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Adjust Viewpoint HeightSTEPSX type 30 in the Height

text box in the ViewshedAnalysis window andpress [Enter] or [Tab]

X press [Test]

To identify cells that are visible from your selectedviewpoints, the viewshed process analyzes the 3Dlines connecting each viewpoint and each cell. If asightline remains entirely above the ground surfacebetween the viewpoint and cell, the cell is visiblefrom that viewpoint.

The viewpoint in the previous exercise is onthe surface at the top of a conical hill risingnear the edge of a flat-floored mountain ba-sin. Although this is the highest elevation onthe hill, the very low slopes on the hilltop block mostof the sightlines to the lower areas immediately sur-rounding the hill. Viewshed extents computed frompoints directly on the surface can be very sensitiveto small differences in elevation around the view-points. You can raise a viewpoint by entering thedesired height above the surface (in meters) in thatpoint’s Height field in the Viewshed Analysis win-dow. A height value of 1 or 2 meters usually gives abetter representation of the area seen by a personstanding at that location, or, con-versely, the area from which aperson or vehicle at that locationwould be visible.

For many viewshed applications thedesired viewpoint is at the top of atower or building some distanceabove the ground. In this exercisethe viewpoint is placed 30 metersabove the ground. As expected, thesize of the viewshed is greatly in-creased by elevating the viewpoint.By running a number of viewshedtests with different heights you candetermine the minimum structureheight required to produce a desiredviewshed.

This DEM has been set todisplay with shaded relief asdescribed earlier in thisbooklet. Once set, theseand other displayparameters are stored withthe DEM and usedautomatically by the displayinterface in any TNTmipsprocess.

Viewshed for a viewpoint 30 metersabove the ground surface.


page 16

Adjust Field of ViewSTEPSX scroll the pointlist in the

Viewshed Analysis to theright to reveal the SweepAngle and View Distancefields

X enter 150 in the ViewDistance field for Point 1

X enter 90 in the SweepAngle field for Point 1

X press [Test]

The Field of View tool, which is ac-tive by default when you start theViewshed process, can be used tolimit the horizontal field of view tobe included in the viewshed analysis for any view-point. You can set the horizontal field of view values(Start Angle, Sweep Angle, and View Distance) bydragging elements of the circular tool graphic in theView or by entering values directly in the respectivefields in the Viewshed Analysis pointlist. The twohorizontal lines of the tool graphic define the StartAngle and Sweep angle; both are measured counter-

clockwise indegrees, with thepositive x-axisdirection corre-sponding to a0-degree start

angle. The position of the arc of the circle definesthe radial View Distance limit, which is measured inraster cells. The mouse cursor changes shape de-pending upon which of these graphic elements it ispositioned closest to, as described in the figure cap-tions below.

You can also set limits onthe vertical field of view for aviewpoint using the DownAngle and Up Angle fields.Both angles are specified indegrees where 0 =horizontal. The Up Anglecan range from 0 to 90degrees, and the DownAngle from 0 to -90degrees.

X press [Run]X use the standard Select

Objects dialog to namea new Project File andan output raster object

When you are ready to create a permanent outputraster with the results of the viewshed analysis, pressthe Run button on the Viewshed Analysis window.

Press the Restore Defaultsicon button to restore thedefault field of view settings.

Drag the arc to change the View Distance(using the pointing hand cursor)

Start Angle (right-arrow cursor)

Sweep Angle(left-arrow cursor)


page 17

Viewshed from Multiple ViewpointsSTEPSX reset the Sweep Angle

for Point 1 to 360X press the Add

icon button on theViewshedAnalysis window

X drag the new Point 2 toa location in the northernhalf of the raster

X set the Height of Point 2to 30

X set the View Distancefor Point 2 to 150

X press [Test]

You can add any number of viewpoints to the analy-sis using the Add icon button on the ViewshedAnalysis window. Each new point is added to theviewpoint list and is initially placed in the center ofthe raster, ready for you to position it in the desiredlocation. You can set viewshed parameters indepen-dently for each viewpoint by editing the settings inthe viewpoint list or by highlighting the point’s listentry and adjusting the point’s Field of View toolgraphic. A joint viewshed for all points is computedwhen you test or run the process.

Viewshed for two viewpoints with viewdistances each set to 150 raster cells.

NOTE: When a new viewpoint is added at the raster center, its view distance is set justlarge enough to include all of the raster area. This ensures that at least parts of thefield of view tool’s view distance arc are visible at full view of the raster for ease ofadjustment. However, when you drag the point to a new location, parts of the rasterwill lie outside the view distance circle. If you want all of the raster area included in theanalysis, you need to increase the view distance appropriately for each new point.

In its default mode, the viewshed computationassumes a “flat-earth” geometry: the surfacedefined by a single elevation is a horizontalplane. This assumption is appropriate forlocal viewshed analysis. If the area you areanalyzing is larger in extent (tens of kilome-ters across or greater), you will achieve moreaccurate results by turning on the Allow forEarth Curvature toggle button.

You can use the SaveViewpoints as Vector iconbutton to save theviewpoints for later reuse inthe process.


page 18

Load Saved ViewpointsSTEPSX press the Load

Viewpoints iconbutton on theViewshed Analysiswindow and chooseLoad from the dropdownmenu

X select vector object VPTS

from the VIEWSHED

Project FileX enter 2.0 in the Height

field for each of the 9viewpoints

X enter 25 in thePercentage ofViewpoints field at thebottom of the ViewshedAnalysis window

The Load Viewpoints icon button allows you to addviewpoints from any geometric object (vector, CAD,or Shape) that contains point elements. The buttonmenu provides options to clear any existing view-points (Load) or add to any existing viewpoints(Append).

When a viewshed is computed from multiple view-points, in the default mode any raster cell that isvisible from at least one of the viewpoints is includedin the viewshed. You can set stricter guidelines forvisibility by changing the value in the Percentage ofViewpoints field (default = 0). This field sets thethreshold percentage of viewpoints that must be vis-ible from any cell in order for that cell to be includedin the viewshed.

X close the Viewshed processby pressing [Exit] when youhave completed this exercise

X press [Test] on theViewshed Analysiswindow


page 19

STEPSX select Raster / Elevation

/ Cut and Fill Analysisfrom the TNTmips menu

X press [DEM1] on the Cut/ Fill window and selectraster FILLED from thePONDS Project File

X press [DEM2] and selectraster PONDS from thesame file

X press [Run] and createan output Project File

X accept the defaultnames for the outputvector object andattached database table

Cut and Fill Volumetric AnalysisIn the Cut and Fill Analysis process you select twoelevation models that have the same size, geographicextents, and cell size. Elevations in the model se-lected as DEM2 are subtracted from those in DEM1.Polygons outlining the areas of net elevation differ-ence are displayed automatically in the View windowat the conclusion of processing.

The Cut and Fill process can be used to assesschanges in landscapes through time due to erosionand deposition or landsliding. In this exercise weinstead compare the DEM of an area with naturaldepressions, many of which contain ponds, with itsdepressionless equivalent (FILLED) produced by theWatershed process. Polygons with positive volumesidentify depressions thathave additional water stor-age capacity.

The VOLUMES table records thevolume difference for eachpolygon, and whether thedifference is positive ornegative.

Turn on the Compute DifferenceRaster toggle button if you wantto save the raster that recordsthe difference in elevation foreach cell location in the input


page 20

Advanced Software for Geospatial Analysis

MicroImages, Inc.11th Floor - Sharp Tower206 South 13th StreetLincoln, Nebraska 68508-2010 USA

Voice: (402) 477-9554 email: [email protected]: (402) 477-9559 internet: www.microimages.com

MicroImages, Inc. publishes a complete line of professional software for advanced geospatialdata visualization, analysis, and publishing. Contact us or visit our web site for detailed prod-uct information.

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.

Indexaspect........................................3,9,10color palette.....................................5,7color shaded relief.................................7,8contrast enhancement.......................4,5,8cut and fill..........................................3,19h i s togram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4relief shading................................6,7,8,15shading raster.................................3,12,13slope.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3,9,10

transparency..................................7,15v i e w p o i n t . . . . . . . . . . . . . . . . . . . . . . . 1 4 - 1 8

field of view..........................16he igh t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

viewshed. . . . . . . . . . . . . . . . . . . . . . . . . . . .3 ,14-18viewshed test...................................14volumes table...................................19w a t e r s h e d . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 , 1 9

TERRAIN

ANALYSIS

analyzing terrain and surfaces

Documents