Abstract:
The 15-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. Vertical elevations are in decimal and
whole units of meters and feet. Elevation values for Alaska are
referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29)
and
... horizontally to the North American Datum of 1927 ( NAD 27) or
NAD 83. The spacing between elevations along profiles is
2-arc-seconds of latitude by 3 arc-seconds of longitude. Each profile
has 451 elevations. DEM data for 15-minute units correspond to the USGS
15-minute topographic quadrangle map series in Alaska. The unit
sizes in Alaska vary depending on the latitude. Units south of 59
degrees N. cover 15- by 20-minute areas, those between 59 degrees and 62
degrees N. cover 15- by 22.5-minute areas, those between 62 degrees
and 68 degrees N. cover 15- by 30-minute areas, and those north of
68 degrees N. cover 15- by 36-minute areas. (All values are
latitude-longitude, respectively.) 
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 15-minute DEM data have rows and columns that are
... staggered and
vary in length. The UTM bounding coordinates form a quadrilateral
rather than a rectangle, so no two sides are parallel to each other.
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. The
USGS dlgv32 product is a Windows 95/NT based freeware viewer that
supports a variety of digital cartographic data. For more
information visit:
http://mcmcweb.er.usgs.gov/
