EM 1110-2-2907
1 October 2003
Datum
In surveying, a reference system for computing or correlating the
results of surveys. There are two principal types of datums: vertical and
horizontal. A vertical datum is a level surface to which heights are
referred. In the United States, the generally adopted vertical datum for
leveling operations is the national geodetic vertical datums of 1929
(differing slightly from mean sea level). The horizontal datum, used as a
reference for position, is defined by: the latitude and longitude of an
initial point, the direction of a line between this point and a specified
second point, and two dimensions which define the spheroid. In the
United States, the initial point for the horizontal datum is located at
Meade's Ranch in Kansas.
Defense Meteorological
A U.S. Air Force meteorological satellite program with satellites circling
Satellite Program (DMSP)
in sun-synchronous orbit. Imagery is collected in the visible- to near-
infrared band (0.4 to 1.1 micrometers) and in the thermal-infrared band
(about 8 to 13 micrometers) at a resolution of about three kilometers.
While some of the data is classified, most unclassified data is available
to civilian users.
DEM--Digital Elevation
The U.S. Geological Survey produces five primary types of digital
Models
elevation model data. They are:
7.5-minute DEM (30- x 30-m data spacing, cast on Universal
Transverse Mercator (UTM) projection or 1- x 1-arc-second data
spacing). Provides coverage in 7.5- x 7.5-minute blocks. Each
product provides the same coverage as a standard USGS 7.5-
minute map series quadrangle. Coverage: Contiguous United
States, Hawaii, and Puerto Rico.
Degree DEM (3- x 3-arc-second data spacing). Provides coverage
in 1- x 1-degree blocks. Two products (three in some regions of
Alaska) provide the same coverage as a standard USGS 1-x 2-
degree map series quadrangle. The basic elevation model is
produced by or for the Defense Mapping Agency (DMA), but is
distributed by USGS in the DEM data record format. Coverage:
United States
30-minute DEM (2- x 2-arc-second data spacing). Consists of four
15- x 15-minute DEM blocks. Two 30-minute DEMs provide the
same coverage as a standard USGS 30- x 60-minute map series
quadrangle. Saleable units will be 30- x 30-minute blocks, that is,
four 15- x 15-minute DEMs representing one half of a 1:100,000-
scale map. Coverage: Contiguous United States, Hawaii.
15-minute Alaska DEM (2- x 3-arc-second data spacing, latitude by
longitude). Provides coverage similar to a 15-minute DEM, except
that the longitudinal cell limits vary from 20 minutes at the
southernmost latitude of Alaska to 36 minutes at the northern most
latitude limits of Alaska. Coverage of one DEM will generally
correspond to a 1:63,360-scale quadrangle.
7.5-minute Alaska DEM (1- x 2-arc-second data spacing, latitude by
longitude). Provides coverage similar to a 7.5-minute DEM, except
that the longitudinal cell limits vary from 10 minutes at the
southernmost latitude of Alaska to 18 minutes at the northernmost
latitude limits of Alaska.
Densitometer
Optical device for measuring the density of photographic
transparencies.
Density, of images
Measure of the opacity, or darkness, of a negative or positive
transparency.
Density, of materials (r)
Ratio of mass to volume of a material, typically expressed as grams per
cubic centimeter.
Glossary-7
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