Thales Navigation Mobile Mapper - Manual (Page 110)

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The most common coordinate systems are Universal Trans-
verse Mercator (UTM) and latitude/longitude, but there are
many variations that are tied to specific regions of the Earth.
MobileMapper allows you to use any of the more common
grid coordinate systems or create your own.
Each regional system is tied to known positions within the
region according to their positional relative to a geodetic da-
tum. According to the Geodetic Glossary (National Geodetic
Survey, National Ocean Service, National Oceanic and At-
mospheric Administration, Rockville, MD, September
1986) p. 54, a geodetic datum is: “A set of constants speci-
fying the coordinate system used for geodetic control, i.e.,
for calculating the coordinates of points on the Earth.” The
key words here are “geodetic control” - the means of estab-
lishing grid or lat/lon coordinates relative to given control
points. The datum is what is “given.” The most common ex-
amples of datums in North America are NAD27 and
NAD83- the North American Datums of 1927 and 1983.
Most countries have their own datums and there are also re-
gional datums for Europe and other parts of the world.
There have also been attempts to create datums for the entire
world. The most widely used is Worldwide Geodetic System
developed in 1984 (“WGS84”). The Global Positioning Sys-
tem, being global, uses WGS84 as a standard datum. All
GPS receivers, including MobileMapper, calculate their po-
sitions in the lat/lon coordinate system tied into the WGS84
datum. Users can then select other coordinate systems to dis-
play their data and other datum to define how the coordinates
are calculated. There are many mathematical algorithms for
“transforming” coordinates from one system to another ac-
cording to various datums.