SPATIAL DATA STRUCTURES
Modeling surface entities such as height, pollution and rainfall pose interesting problems in GIS.
Digital Terrain Models (DTMs) are surface models created from information on height, slope, aspect,
breaks in slope and other topographic features
A DTM is a digital data set used to model a topographic surface (representing height data).
MODELING SURFACES
Modeling a surface accurately would entail storing an almost infinite number of observations – an
impossibility, of course
Using a finite number of observations, a surface model can be derived.
The ‘resolution’ of the DTM is determined by the frequency of observations.
DTMs are created, then, from a series of either regularly or irregularly spaced x, y, and z data points.
EXAMPLES OF SURFACE TYPES
MODELING SURFACES
DTMs are derived from a number of data sources including:
Contours and spot heights from topograhic maps
Satellite images
Field surveys using GPS
CONTOURS AND SPOT HEIGHTS
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STEREOSCOPIC AERIAL PHOTOGRAPHS
SATELLITE IMAGERY
Radar or laser scanning sensors
SAR – Synthetic Aperture Radar
LiDAR – Light Detection and Ranging
SRTM – Shuttle Radar Topography Mission
Resolution: a few meters to 90 meters
Provide elevation data in DEM (Digital Elevation Model) for direct input in GIS
SATELLITE IMAGERY
RASTER APPROACH TO DTM
In a raster GIS, a DTM is a grid of height values.
Each cell contains a single value representative of the height of the terrain that is covered by that cell.
This is a simple raster DTM referred to as a DEM – Digital Elevation Matrix.
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There
is no other information other than the elevation or height above the datum.
more complex the terrain, the more data points are required – finer resolution
The simpler the terrain and fewer data points are required – coarser resolution
The
SIMPLE AND COMPLEX RASTER DTM
VECTOR APPROACH TO DTM
A vector DTM mimics the raster DTM by using regularly spaced set of spot heights to represent
terrain surfaces.
Triangulated irregular network (TIN) is a more complex form of a vector DTM
A TIN is used to create a DTM from either regular or irregular surface height data.
DIGITAL TERRAIN MODELS
DEM AND TIN MODELS FOR REGIONS WITH VARYING COMPLEXITY
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TIN MODELS
The main advantage of the TIN model is efficiency of data storage.
Additionally, since TINs are created from irregularly spaced points, it can easily model complex
terrain: more points for mountainous areas and fewer points for flatter regions.
TERRAIN MODELING
There are five common terrain mapping techniques:
Contouring
Vertical profiling
Hill shading
Hypsometric tinting
Perspective view
CONTOURING
The most common method for terrain mapping.
Contour lines connect points of equal elevation
Contour interval represents the vertical distance between contour
Base contour is the contour line from which all contouring starts
lines
CONTOUR MAP BASICS
The arrangement and pattern of contour lines reflect the topography
Close lines represent steep slopes, while contour lines that are widely spaced, represent gentle slopes.
Contour lines will bend upstream when crossing a stream valley.
Contour lines do not cross each other or stop in the middle of a map.
Closed contour lines can represent either a mountain top or a basin.
Basins are shown as closed contour lines with hachure lines pointing to the lower elevation in the
center.
VERTICAL PROFILING
A vertical profile shows the changes in elevation along a line, such as a hiking trail, a road or a
stream.
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VERTICAL PROFILE
HILLSHADING
Also known as shaded relief, hill shading simulates how the terrain looks with the interaction
between sunlight and surface features.
A mountain slope facing into oncoming light will appear bright, while the slope facing away will be
dark
Hill shading helps viewers recognize the shape of landform features
DEM AND HILLSHADE EXAMPLES
HILLSHADING EXAMPLES
HILLSHADING TECHNIQUES
Four factors control the visual effect of hill shading:
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(1) The Sun’s azimuth – the direction of incoming light, ranging from 0 (due north) to 360 in a
clockwise direction.
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Typically, the default azimuth is 3150
0
0
(2) The Sun’s altitude – the angle of incoming light measured above the horizon between 0 and 90

(3) The slope of the surface – ranging between 00 – 900
0
0
(4) The aspect of the surface – ranging between 0 - 360

SUN’S AZIMUTH AND ALTITUDE
SURFACE SLOPE AND ASPECT
ASPECT AND SLOPE FACTORS
The accuracy of slope and aspect measures can influence the performance of models
The most important factor is the resolution of the DEM used for deriving slope and aspect
The quality of the DEM also influences slope and aspect measures
Dependent on software package and input data (ground control points)
Variation in computing algorithms affects slope and aspect measures
Local topography can also be a factor in estimating slope and aspect.
Steeper slopes may create errors in slope estimates, but gentler slopes create errors in aspect
estimates.
Slope and aspect are basic elements for analyzing and visualizing landform characteristics.
Slope and aspect are important in studies of watershed units, landscape units and other measures.
Slope and aspect can also assist in solving problems in forest inventory estimates, soil erosion, habitat
suitability, site analysis and others
HYPSOMETRIC TINTING
Hypsometry depicts the distribution of the Earth’s mass with elevation.
Hypsometric tinting, layer tinting, applies color symbols to different elevation zones.
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HYPSOMETRIC TINTING
PERSPECTIVE VIEW
Perspective Views are 3-D views of the terrain
The terrain has the same appearance as it would have it viewed at an angle from an airplane
Four parameters control appearance:
0
0
(1) Viewing azimuth: direction from observer to surface (0 -360 clockwise)
0
0
(2) Viewing angle: angle measured from horizon to altitude of observer (0 -90 )
0
3D effect reaches maximum as angle approaches 0
0
3D effect reaches minimum as angle approaches 90
(3) Viewing distance: distance between the viewer and surface
(4) z-scale: the ratio between the vertical scale and horizontal scale
Also called the vertical exaggeration factor, the z-scale is useful for highlighting minor landform
surfaces
PERSPECTIVE VIEW
3D PERSPECTIVE VIEW
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PLAN OBLIQUE RELIEF FROM TOPOGRAPHIC MAP
Big Horn Basin, Wyoming. On left, a drawing of region based on topography and on the right a plan
oblique relief based on SRTM
3-D DRAPING
To make perspective views more realistic, layers such as hydrographic features, land cover,
vegetation and roads can be superimposed on the views.
3D DRAPING
TERRAIN MAPPING AND ANALYSIS USING ARCGIS
Spatial Analyst and 3D Analyst extensions in ArcGIS are tools used for terrain mapping and analysis.
Using ArcToolbox, the Spatial Analyst Tool/Surface toolset includes tools for aspect, contour,
curvature, hillshade, and slope.
The 3D Analyst Tools/Conversion toolset has tools for conversion between TIN, raster, and features.
The 3D Analyst Tools/TIN Surface toolset has tools for deriving aspect, contour, and slope features
layers directly from TINs.
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