Digital Elevation Models And Relief Models DEM

Part 1: The Underlying Elevation Data DEM

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 DEM

Digital Elevation Model: What it looks like 67 56 49 46 50 12 11 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 34 Lay a grid over some part of the world and find the average elevation in each cell DEM

Cell Definition cell size 50 67 56 49 46 50 12 11 53 44 37 38 48 58 55 22 31 24 61 47 21 16 19 34 (cell value) cell The size of a cell is in: meters, feet, degrees, or arc seconds DEM

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” (” stands for arc second – about 30m --varies) National Elevation Dataset 3" (100m) DEMs from 1:250,000 scale maps 30“ (1,000m) DEM of the earth (GTOPO30) DEM

Primary Data Source Generation: Space Shuttle DEM

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 DEM

DEM

Santa Barbara, California Note vertical exaggeration http://srtm.usgs.gov/srtmimagegallery/index.html DEM

San Andreas Fault, California Distance to Horizon: 73 kilometers (45.3 miles) Location: 35.42 deg. North lat., 119.5 deg. West lon. View: Toward the Southeast Date Acquired: February 16, 2000 SRTM, December 14, 1984 Landsat Image: NASA/JPL/NIMA Original Caption Released with Image: The 1,200-kilometer (800-mile) San Andreas is the longest fault in California and one of the longest in North America. This perspective view of a portion of the fault was generated using data from the Shuttle Radar Topography Mission (SRTM), which flew on NASA's Space Shuttle last February, and an enhanced, true- color Landsat satellite image. The view shown looks southeast along the San Andreas where it cuts along the base of the mountains in the Temblor Range near Bakersfield. The fault is the distinctively linear feature to the right of the mountains. To the left of the range is a portion of the agriculturally rich San Joaquin Valley. In the background is the snow-capped peak of Mt. Pinos at an elevation of 2,692 meters (8,831 feet). The complex topography in the area is some of the most spectacular along the course of the fault. To the right of the fault is the famous Carrizo Plain. Dry conditions on the plain have helped preserve the surface trace of the fault, which is scrutinized by both amateur and professional geologists. In 1857, one of the largest earthquakes ever recorded in the United States occurred just north of the Carrizo Plain. With an estimated magnitude of 8.0, the quake severely shook buildings in Los Angeles, caused significant surface rupture along a 350-kilometer (220-mile) segment of the fault, and was felt as far away as Las Vegas, Nev. This portion of the San Andreas is an important area of study for seismologists. For visualization purposes, topographic heights displayed in this image are exaggerated two times. The elevation data used in this image was acquired by SRTM aboard the Space Shuttle Endeavour, launched on February 11, 2000. SRTM used the same radar instrument that comprised the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) that flew twice on Endeavour in 1994. SRTM was designed to collect three-dimensional measurements of Earth's land surface. To collect the 3-D SRTM data, engineers added a mast 60 meters (about 200 feet) long, installed additional C-band and X-band antennas, and improved tracking and navigation devices. The mission is a cooperative project between the NASA, the National Imagery and Mapping Agency (NIMA) of the U.S. Department of Defense, and the German and Italian space agencies. It is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif, for NASA's Earth Science Enterprise, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.   http://srtm.usgs.gov/srtmimagegallery/index.html DEM

(SRTM) Salt Lake City, Utah http://srtm.usgs.gov/srtmimagegallery/index.html DEM

Mt Kilimanjaro, Tanzania http://srtm.usgs.gov/srtmimagegallery/index.html DEM

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. DEM

Secondary Data Source Generation: Using USGS Topographic Maps DEM

200 Meter Mesh (Universal Transverse Mercator Coordinates) 1km 1km DEM

100 Meter Mesh (UTM Coordinates) 1km 100m 1km DEM

30 Meter Mesh Standard for 1:24,000 Scale Maps DEM

DEM Lattice Points DEM

DEM Cell Stores Elevation at Lattice Point

DEM Elevations 720 720 Contours 740 720 700 680 740 720 700 680 DEM

Comparison Cell Size 30m 100m Note difference in detail DEM

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 DEM

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/ DEM

Part 2: Representation of elevation data DEM