Satellite data can also be used to create elevation, or topographic data using stereoscopic pairs, as discussed above under SPOT. Radar sensor data can also be a source of topographic information, as discussed in Terrain Analysis. However, most available elevation data are created with stereo photography and topographic maps.
ERDAS IMAGINE software can load and use:
- USGS DEMs
Most elevation data are in arc/second format. Arc/second refers to data in the Latitude/Longitude (Lat/Lon) coordinate system. The data are not rectangular, but follow the arc of the Earth’s latitudinal and longitudinal lines.
Each degree of latitude and longitude is made up of 60 minutes. Each minute is made up of 60 seconds. Arc/second data are often referred to by the number of seconds in each pixel. For example, 3 arc/second data have pixels which are 3 × 3 seconds in size. The actual area represented by each pixel is a function of its latitude. The figure below illustrates a 1° × 1° area of the Earth.
A row of data file values from a DEM or DTED file is called a profile. Profiles of DEM and DTED run south to north, that is, the first pixel of the record is the southernmost pixel.
In the figure above, there are 1201 pixels in the first row and 1201 pixels in the last row, but the area represented by each pixel increases in size from the top of the file to the bottom of the file. The extracted section in the example above has been exaggerated to illustrate this point.
Arc/second data used in conjunction with other image data, such as TM or SPOT, must be rectified or projected onto a planar coordinate system such as UTM.
DEMs are digital elevation model data. DEM was originally a term reserved for elevation data provided by USGS, but it is now used to describe any digital elevation data.
DEMs can be:
- purchased from USGS (for US areas only)
- created from stereopairs (derived from satellite data or aerial photographs)
See Terrain Analysis for more information on using DEMs.
In 2006, USGS began offering National Elevation Dataset (NED). NED has been developed by merging the highest-resolution elevation data available across the United States into a seamless raster format. The dataset provides seamless coverage of the United States, Hawaii, Alaska, and the island territories.
There are two types of historic DEMs that are most commonly available from USGS:
- 1:24,000 scale, also called 7.5-minute DEM, is usually referenced to the UTM coordinate system. It has a spatial resolution of 30 × 30 m.
- 1:250,000 scale is available only in Arc/second format.
Both types have a 16-bit range of elevation values, meaning each pixel can have a possible elevation of -32,768 to 32,767.
DEM data are stored in ASCII format. The data file values in ASCII format are stored as ASCII characters rather than as zeros and ones like the data file values in binary data.
DEM data files from USGS are initially oriented so that North is on the right side of the image instead of at the top. ERDAS IMAGINE rotates the data 90 degrees counterclockwise as part of the Import process so that coordinates read with any ERDAS IMAGINE program are correct.
DTED data are produced by National Geospatial-Intelligence Agency (NGA) and are available only to US government agencies and their contractors. DTED format is described in military specification MIL-PRF-89020B.
There are two types of DTED data available:
- DTED 1 — a 1° × 1° area of coverage
- DTED 2 — a 1° × 1° or less area of coverage
Both are in Arc/second format and are distributed in cells. A cell is a 1° × 1° area of coverage. Both have a 16-bit range of elevation values.
Like DEMs, DTED data files are also oriented so that North is on the right side of the image instead of at the top. ERDAS IMAGINE rotates the data 90° counterclockwise as part of the Import process so that coordinates read with any ERDAS IMAGINE program are correct.
Using Topographic Data
Topographic data have many uses in a GIS. For example, topographic data can be used in conjunction with other data to:
- calculate the shortest and most navigable path over a mountain range
- assess the visibility from various lookout points or along roads
- simulate travel through a landscape
- determine rates of snow melt
- orthocorrect satellite or airborne images
- create aspect and slope layers
- provide ancillary data from image classification
See Terrain Analysis for more information about using topographic and elevation data.