Digital Elevation Models And Relief Models

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Part 1:The Underlying Elevation Data

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Digital Elevation Models are:• Data files that contain the elevation of the terrain over a specified area,

usually at a fixed grid interval • The intervals between each of the grid points will always be referenced to

some geographical coordinate system. (e.g. latitude-longitude, UTM, SP). • The closer together the grid points are located, the more detailed the

information will be in the file. The details of the peaks and valleys in the terrain will be better modeled with a small grid spacing than when the grid intervals are very large. Elevations other than at the specific grid point locations are not contained in the file. As a result peak points and valley points not coincident with the grid will not be recorded in the file.

• The DEM file also does not contain civil information such as roads or buildings. It is not a scanned image of the paper map (graphic). It is not a bitmap. The DEM does not contain elevation contours, only the specific elevation values at specific grid point locations.

(SEE NEXT SLIDE FOR EXAMPLE)

source: http://www.softwright.com/dem.html

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67 56 49 46 50

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53 44 37 38 48

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Digital Elevation Model: What it looks like

Lay a grid over some part of the world and find the average elevation in each cell

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50

cell size

cell

(cell value)

Cell Definition

The size of a cell is in either: meters, feet, degrees, or arc seconds

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DEM Data Sources

Primary Data Sources:• Shuttle Radar Topography Mission (SRTM) or

other airborne sensorsSecondary Sources from existing maps:• 30m DEMs from 1:24,000 scale map• 1” (arc second ) National Elevation Dataset • 3" (100m) DEMs from 1:250,000 scale maps• 30" DEM of the earth (GTOPO30)

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Primary Data Source Generation:Space Shuttle

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Shuttle Radar Topography Mission (SRTM)

• 1 arc-second elevation data for the United States, 3 arc-second data for the globe

• Produced by radar measurements from a Shuttle mission, Feb 11-22, 2000

• http://srtm.usgs.gov/data/obtainingdata.html

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http://srtm.usgs.gov/srtmimagegallery/index.html

Santa Barbara, California

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http://srtm.usgs.gov/srtmimagegallery/index.html

San Andreas Fault, California

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http://srtm.usgs.gov/srtmimagegallery/index.html

Salt Lake City, Utah

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http://srtm.usgs.gov/srtmimagegallery/index.html

Mt Kilimanjaro, Tanzania

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Interferometry used by SRTM

In interferometry, two images are taken from different vantage points of the same area. The slight difference in the two images allows scientists to determine the height of the surface.

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Secondary Data Source Generation:

Using USGS Topographic Maps

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200 Meter Mesh (Universal Transverse Mercator Coordinates)

1km

1km

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100 Meter Mesh (UTM Coordinates)

100m

100m

1km

1km

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30 Meter MeshStandard for 1:24,000 Scale Maps

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DEM Lattice Points

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DEM Cell Stores Elevation at Lattice Point

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DEM Elevations

Contours

720

700

680

740

680700720740

720 720

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Cell Size30m 100mComparison

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30m DEMs

• Best resolution standardized data source available for the US

• Coverage of the country is incomplete• Data by 7.5’ map sheets in UTM projection• Link for US

http://edcwww.cr.usgs.gov/Webglis/glisbin/guide.pl/glis/hyper/guide/usgs_dem

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National Elevation Dataset

• Seamless 1” DEM for the US in 1° x 1° blocks

• Compiled by synthesizing the 30m DEM’s from 1:24,000 scale maps

• Link to website http://edcnts12.cr.usgs.gov/ned/

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Part 2:Representation of elevation data

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