Download - Geography 360 Principles of Cartography
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Geography 360Principles of Cartography
May 19, 2006
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Outlines: Digital Elevation Model
1. How is DEM different from map display?• DEM is data structure, not map
2. What is DEM?• Digital representation of elevation• Data structure for representation stored in the
computer
3. Three commonly used DEM• Contour• TIN• Grid (most of time considered equivalent to DEM)
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Phenomenon, data, map
• Distinguish these
Phenomenon Spatial data structure
Spatial dimension
Map display
#1 Population Point Polygon
0D2D
Dot map, Proportional symbol map, Choropleth map, Dasymetric map
#2 Bus route LineGraph
1D Flow map
#3 Land use Polygon 2D Categorical map
#4 Elevation GridTIN, Contour
3D Contour, shaded relief map, fishnet map
Refer back to SDTS spatial data concepts
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DEM
• Data representation is different from map display• How is elevation (or the third dimension) represented in
database?• DEM is a rubric term that refers to data structure used to
represent elevation in the computer• There are three widely used DEM: (1) contour (2) TIN (3)
grid • DEM, narrowly defined, refers to gridded form for
representing elevation • DEM is data representation, not map display• Understanding DEM is important because it serves as
the source of map display (you will create surface map from DEM)
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DEM
• Data structure for elevation
• Representing pyramid…
Source: NCGIA
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Point dataContour
Triangulated Irregular Network (TIN)
Grid
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Earth’s topography in different data structure
Source: Bolstad 2005
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Source: Nyerges
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How is DEM generated?
• 1) From paper map (most common)• Vectorization of contour lines printed on the map (it can
be seen as scattered points which has varying elevation values)
• If necessary, hydrogrphy features (e.g. river) are added to enhance the quality of DEM
• At grid points, elevation values are interpolated from measured points
• 2) From photogrammetry• Extraction of elevation from photographs either manually
(using stereoplotter) or automatically (using instruments)
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TIN
• TIN = Triangulated Irregular Network
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TIN
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How is DEM used?
• DEM is useful in (1) data storage (2) map display (3) further analysis (e.g. deriving slope, aspect, viewshed, computing least cost path, and much more)
• The use of DEM not limited to representing elevation - you can apply the concept of DEM to any continuous surface (statistical surface such as demographic structure, geological profile, and so on)
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Where do I obtain DEM?
• EROS Data Center
• You may have to convert DEM to the format compatible with software (the conversion capability is available in most commercial GIS package)
• You can create your own DEM or surface map by interpolating point data provided that the phenomenon is continuous