Digital Elevation Models And
Relief Models
DEM
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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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Digital Elevation Model:
What it looks like
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53 34 12 11 12
Lay a grid over some part of the world and find the average
elevation in each cell
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Cell Definition
cell size
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50
(cell value)
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53 34 12 11 12
cell
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 sensors
Secondary 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 1122, 2000
• http://srtm.usgs.gov/da
ta/obtainingdata.html
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Santa Barbara, California
http://srtm.usgs.gov/srtmimagegallery/index.html
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San Andreas Fault, California
DEM
http://srtm.usgs.gov/srtmimagegallery/index.html
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Salt Lake City, Utah
http://srtm.usgs.gov/srtmimagegallery/index.html
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Mt Kilimanjaro, Tanzania
http://srtm.usgs.gov/srtmimagegallery/index.html
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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
1km
100m
1km
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30 Meter Mesh
Standard 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
720
720
Contours
740
720
700
680
740 720 700
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Comparison
30m
Cell Size
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100m
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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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