| # GRASS Help System

## 7.5 MINUTE DEM DATA

The U.S. Geological Survey (USGS) distributes 1-degree and 7.5-minute digital elevation models (DEMs). 1-degree DEMs have a resolution of 100 meters, while 7.5-minute DEMs have a resolution of 30 meters. Both are distributed on 1/2 inch, 9-track magnetic tapes. Data are referenced using the Universal Transverse Mercator (UTM) coordinate system.

Characteristics of 7.5 Minute DEMs:
* The unit of coverage is a 7.5-minute x 7.5-minute block. Each
block provides the same coverage as a standard USGS 1:24,000
scale map series quadrangle.

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

* Elevation points are spaced at intervals of 30 meters along and
between each profile. This provides a 30 meter resolution
for the data file.

* Elevations are in either meters or feet relative to mean sea.
level. DEMs that are generated of low-relief terrain, or from
contour maps with intervals of 5 feet or less, are generally
recorded in feet. DEMs generated of moderate-to-high-relief
terrain, or from contour maps with intervals in excess of 5 feet,
are generally recorded in meters.

* Price is $100.00 for each 7.5 minute x 7.5 minute block.

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Automatically created on: Thu Mar 16 08:45:08 2006