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US Digital Topography for GIS

U.S. Digital Topography for GIS provides a set of full color, shaded relief maps which can be imported into and geo-referenced in GIS software such as Environmental Sytems Research Institute's ArcView and ARC/INFO. These raster maps constitute a highly detailed topographic base over which vector data features such as streams, roads, etc. can be overlayed. The map images in this collection have been computed from a 6 arc second subset of the USGS 3 arc second, gridded elevation database.

Coverage & Organization

The collection covers the entire United States with the exception of Alaska. Each state has its own directory. Within that directory are all one-by-one degree quadrangles of which the state is composed. There is also one screen-sized map which shows the topography of the entire state.

Image size

One-by-one degree maps measure 8.33 by 8.33 inches on a 72 dpi screen. They are composed of 600 rows and 600 columns. On one of these maps the distance, from bottom to top, is 69 miles. Image size of state screen- sized maps varies.

Format

Uncompressed TIFF ISO 9660 format CD-ROM. The collection ships in a 3 CD-ROM set complete with instruction booklet.

Map Scale

The scale of one-by-one degree map images is 1:530,000. That of state maps varies. In general, however, state maps provide excellent resolution at scales of 1:1,000,000 to 1:3,000,000.

Identification

One-by-one degree map tiles are identified by the latitude and longitude of their southeast corners. GIS applications identify the tile automatically and position the relief maps appropriatly.

Color and density

RGB 24 bits per pixel, 72 pixels per inch. This means that two or more one-by-one degree map tiles may be assembled to create larger maps either in GIS or image processing software.

Projection & Map units

Geographic projection; Decimal degrees units

Download a sample of California - this is a one degree by one degree quadrangle with geo-referencing suitable for viewing within GIS software.

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