# Terrain Mapping and Analysis by gR1AvP38

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```									Terrain Mapping and
Analysis
Chapter 12
Introduction
 Terrain mapping
 Land surface is 3-D
 Elevation data or z-data is treated as a
cell value or a point data attribute rather
than as a coordinate.
Data for Terrain Mapping and Analysis
   Digital Elevation Model (DEM)
   Array of elevation points
   7.5 minute USGS quads into 4 levels
   Level 1 RMS 7-15 meters
   Level 2 RMS of ½ contour interval
   Level 3 RMS of 1/3 contour interval not to
exceed 7 meters
   What happened to Level 4?
   Relative and absolute errors
Data for Terrain Mapping and Analysis
   Triangulated Irregular Network (TIN)
   Series of non-overlapping triangles
   Elevation values are stored at nodes
   Irregular distribution
   Sources: DEMs, surveyed elevation points,
contour lines, and breaklines
   Breaklines are line features that represent
changes of the land surface such as streams,
Data for Terrain Mapping and Analysis
   Triangulated Irregular Network (TIN)
   Not every point in DEM is used
   Only points most important
   VIP (Very Important Points) algorithm
   Maximum z-tolerance algorithm
   Delaunay triangulation: all nodes are
connected to their nearest neighbor to form
triangles which are as equi-angular as
possible.
   Borders are a problem
   Go beyond study area and clip to make best
Terrain Mapping
 Contouring is most common method for
terrain mapping
 Contour lines connect points of equal
elevation (isolines)
 Contour intervals represent the vertical
distance between contour lines.
 Arrangement of contour lines reflect
topography
Terrain Mapping
   Vertical profile shows changes in elevation
along a line, such as a hiking trainl, road
or stream.
Terrain Mapping
 Attempts to simulate how the terrain looks
with the interaction between sunlight and
surface features.
 Helps viewers recognize the shape of land-
form features on a map.
 Digital shaded-relief map of US
Terrain Mapping
   Four factors control the visual effect of
   Sun’s azimuth is direction of incoming light (0
to 360°)
   The sun’s altitude from horizon (0-90°)
   Surface slope (0-90°)
   Surface aspect (0 to 360°)
Terrain Mapping
   Hypsometric tinting
   Applies different color symbols to represent
elevation zones.
Terrain Mapping
   Perspective View
   Perspectives are 3-D views of the terrain
wherein the appearance is as viewed from an
airplane.
   Viewing azimuth (0 to 360°)
   Viewing angle (0-90°)
   Viewing distance
   Z-scale is ratio between he vertical scale and
the horizontal scale (exaggeration factor)
   3-D draping of vector information
Terrain Analysis
 Slope measures the rate of change of
elevation at a surface location
 Aspect is the directional measure of the
slope (degrees- 4 or 8 directions)
 Important for analyzing an visualizing
landform characteristics
 Accuracy an issue
 If you want to try, use the worked
examples in the text with Excel
Terrain Analysis
 Surface curvature: convex or concave
 Viewshed analysis
   Viewshed refers to the areas of the land
surface that are visible from an observation
point or points.
   Watershed analysis
   Watershed is an area that drains water and
other substances to a common outlet
Terrain Analysis
   Watershed analysis
   Requires three data sets in raster format
   Filled elevation grid
   Flow direction grid
   Flow accumulation grid
Grid vs TIN
 Different algorithms and type of output
 Can be converted from TIN to grid or grid
to TIN
 TIN has flexibility of input sources: DEM,
breaklines, contour lines, GPS data and
survey data as well as user added
elevation points.
 Elevation grid is fixed with a given cell size
Grid vs TIN
 Computational efficiency with grid
 TIN gives sharper image
 How are they built and used?

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