Geography 360 Principles of Cartography

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Geography 360Principles of Cartography

• May 19, 2006

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Outlines Digital Elevation Model

• How is DEM different from map display?
• DEM is data structure, not map
• What is DEM?
• Digital representation of elevation
• Data structure for representation stored in the
  computer
• Three commonly used DEM
• Contour
• TIN
• Grid (most of time considered equivalent to DEM)

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Phenomenon, data, map
Refer back to SDTS spatial data concepts

• Distinguish these

Phenomenon Spatial data structure Spatial dimension Map display
1 Population Point Polygon 0D 2D Dot map, Proportional symbol map, Choropleth map, Dasymetric map
2 Bus route Line Graph 1D Flow map
3 Land use Polygon 2D Categorical map
4 Elevation Grid TIN, Contour 3D Contour, shaded relief map, fishnet map
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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 data
Contour
Grid
Triangulated Irregular Network (TIN)
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Earths 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