# GRASS Help System

## CHARACTERISTICS OF DMA'S DTED-1 ELEVATION DATA

A digital elevation data file consists of a sampled array of elevations for ground positions that are generally at regularly-spaced intervals. The Defense Mapping Agency (DMA) produces an elevation product called Digital Terrain Elevation Data (DTED). DTED is available in two levels, DTED-1 and DTED-2. The level refers to the resolution of the elevation data. The resolution is the amount of space between each sample elevation point in the data file. DTED-1 has a resolution of roughly 100 meters, while DTED-2 has a resolution of roughly 30 meters.

Although Both the DMA and the USGS distribute DTED data, each distributes it in a different tape format. The program you use to extract DTED data from tape will therefore depend upon your source of the data (DMA or USGS).

DTED-1 characteristics are given below:
The unit of coverage is a 1-degreeX 1-degree block. This
corresponds to the east or west half of a 1:250,000 USGS map
sheet. However, these blocks are referenced by their
southwest corner coordinates (Lat., Long.) rather than by
the names of the corresponding USGS map sheets.

* Coverage is available for many regions worldwide, although
predominantly for those within the Northern Hemisphere.

* The data are ordered from south to north in profiles that are
ordered from west to east.

* Elevation points along each profile are spaced at intervals of
3 arc-seconds (roughly 100 meters). This provides for
a 100 meter resolution data file. The information content
is approximately equivalent to the contour information on a
1:250,000 scale map.

* Spacing of the elevation points between each profile varies by
latitude. From 0 degrees N to 50 degrees N, the spacing is
3 arc-seconds. From 50 degrees N to 70 degrees N, the spacing
is 6 arc-seconds. North of 70 degrees N, the spacing is
9 arc-seconds.

* Elevations are stated in meters relative to mean sea level.

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Automatically created on: Thu Oct 14 02:12:26 2004