1 October 2003
datums have evolved, each developed by the measurement of different aspects of the
Earth's surface. Models are occasionally updated with the use of new technologies. For
example, in 1984 satellites carrying GPS (global position systems) refined the World
Geodetic System 1927 (WGS-27); the updated datum is referred to as WGS84 (World
Geodetic System1984). Satellite data collected prior to 1984 may have coordinates
linked to the WGS-27 datum. Georeferencing coordinates to the wrong datum may re-
sult in large positional errors. When working with multiple images, it is therefore im-
portant to match the datum for each image.
b. Image processing software provide different datums and will allow users to con-
vert from one datum to another. To learn more about geodetic datums go to
5-8 Image Projections.
a. Many projects require precise location information from an image as well as geo-
coding. To achieve these, the data must be georeferenced, or projected into a standard
coordinate system such as Universal Transverse Mercator (UTM), Albers Conical Equal
Area, or a State Plane system. There are a number of possible projections to choose
from, and a majority of the projections are available through image processing software.
Most software can project data from one map projection to another, as well as unpro-
jected data. The latter is known as rectification. Rectification is the process of fitting the
grid of pixels displayed in an image to the map coordinate system grid (see Paragraph 5-
b. The familiar latitude and longitude (Lat/Long) is a coordinate system that is ap-
plied to the globe (Figure 5-2). These lines are measured in degrees, minutes, and sec-
onds (designated by o, ', and " respectively). The value of one degree is given as 60 min-
utes; one minute is equivalent to 60 seconds (1o = 60'; 1'= 60"). It is customary to
present the latitude value before the longitude value.
5-9 Latitude. Latitude lines, also known as the parallels or parallel lines, are perpen-
dicular to the longitude lines and encircle the girth of the globe. They are parallel to one
another, and therefore never intersect. The largest circular cross-section of the globe is at
the equator. For this reason the origin of latitude is at the equator. Latitude values in-
crease north and south away from the equator. The north or south direction must be re-
ported when sighting a coordinate, i.e., 45oN. Latitude values range from 0 to 90o,
therefore the maximum value for latitude is 90o. The geographic North Pole is at 90oN
while the geographic South Pole is at 90oS