# GRASS Help System

## DEM DATA PRODUCTION METHODS

A digital elevation data file consists of a sampled array of elevations for ground positions that are generally at regularly-spaced intervals. USGS distributes 1-degree and 7.5-minute DEM data.

1-degree DEM data are actually produced (as DTED-1 data) by the Defense Mapping Agency (DMA). DTED data are produced using cartographic and photographic sources. For more information on DEM/DTED data, see "National Mapping Program, Technical Instructions: Data Users Guide 5", p. 5). Cartographic sources include any map series from 1:24,000 scale to 1:250,000 scale. The hypsographic features (contours, ridge lines, spot elevations) and prominent hydrographic features (drainage lines, lakes) are digitized or scanned and then processed into the required matrix format (profiles) of elevation points.

Elevation data are derived from photographic sources using manual and automated stereophotogrammetric equipment. The elevations are weighted with additional information such as drainage lines, ridge lines, water and spot elevations and processed into the required matrix format (profiles) of elevation points.

7.5-minute DEM data are produced either from contour overlays that have been digitized, or from automated or manual scanning of National High Altitude Photography program (NHAP) quad-centered photos (Ibid., p. 3). The USGS has used four processes to collect the digital elevation data from the above sources for production of the 7.5 minute DEM's: (1) the Gestalt Photo Mapper II (GPM2), (2) manual profiling from photogrammetric stereomodels, (3) stereomodel digitizing of contours, and (4) derivation from DLG hypsography and hydrography categories (Ibid., p. 3). Information on these methods is available from the USGS, in: "National Mapping Program, Technical Instructions: Data Users Guide 5" -- Digital Elevation Models.

Private companies can also produce digital terrain models (DTM) from contour plates and stereoscopic air photos. See "DTM Production Methods" for information on DTM production.

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Automatically created on: Mon Apr 4 09:21:45 2005