Abstract:
The 7.5-minute digital elevation model (DEM) data are digital
representations of cartographic information in a raster form. The
DEMs consist of an array of elevations for ground positions at
regularly spaced intervals. The data are produced in 7.5- by
7.5-minute blocks from either digitized cartographic map contour
overlays of a scanned National Aerial Photography Program (NAPP)
photographs. The
... DEM data are stored as profiles with a 10- or
30-meter square grid spacing along and between each profile.
Approximate file sizes are 9.9 megabytes for a 10-meter resolution
and 1.1 megabytes for a 30-meter horizontal resolution. Data
profiles for this product do not always have the same number of
elevations because of the variable angle between true north and grid
north in the Universal Transverse Mercator (UTM) projection
coordinate system. The DEM data for 7.5-minute units correspond to
the USGS 7.5-minute topographic quadrangle map series for all of the
United States and its territories except Alaska. 
Purpose:
DEMs can be used as source data for digital orthophotos, and for
earth science analysis as layers in geographic information systems.
DEMs can also serve as tools for volumetric analysis, for site
location of towers, or for drainage basin delineation. These data
were collected as part of the National Mapping Program.
Supplemental_Information:
The 7.5-minute DEM data have rows and columns that
... vary in length
and are staggered. The UTM bounding coordinates form a quadrilateral
in which no two sides are parallel to each other, rather than a
rectangle. If a rectangle is required for the user's application,
the user will need to pad out the uneven rows and columns with
blanks or flagged data values. Some software vendors have
incorporated this function into their software for input of standard
formatted USGS DEMs. USGS dlgv32--Windows 95/NT based freeware
viewer that supports a variety of digital cartographic data. For more
information visit:http://mcmcweb.er.usgs.gov/
The USGS has entered into an agreement with the GeoCommunity to serve updated,
SDTS format, DEMs through the GIS Data Depot at
&http://www.gisdatadepot.com& for no charge. The 7.5' SDTS DEM data
are also available for purchase via premium (fast) download and on
CD-ROM from GeoCommunity. The decision to provide SDTS DEMs through
the GIS Data Depot is consistent with the USGS goal of making more
public domain data available at no charge while enabling the private
sector to have a greater role in the distribution of USGS data. SDTS
DEMs placed on media in response to customer orders will be sold by
the GeoCommunity at a price less than the USGS currently charges.
The GeoCommunity will continue to receive SDTS versions of the DEMs
as they are made available by the USGS. Native format DEMs are
still available for sale from USGS on various media and via ftp. If you
have additional questions or comments, please contact sdts@usgs.gov.
Please monitor the SDTS Home Page for further news and updates on
this issue.&http://mcmcweb.er.usgs.gov/sdts&
