Garmin GPS 60 - Manual (Page 81)

Map Datums are based on a math-
ematical model of the Earth.
Position
Formats Map Datums
What are Map Datums, and which should I use?
A datum is a mathematical model of the Earth which approximates
the shape of the Earth, and enables calculations to be carried out in a
consistent and accurate manner. The datum is physically represented by
a framework of ground monuments (i.e. trig. stations) whose positions
have been accurately measured and calculated on this reference surface.
Lines of latitude and longitude on a map or chart are referenced to a
specific map datum. Every chart has a map datum reference and the GPS
60 can be set to match most of those commonly used.
If you are navigating and comparing the GPS coordinates to a map,
chart or other reference, the map datum in the GPS unit should be set to
the same datum as the map or chart to insure the most accurate naviga-
tion.
What is a Position Format, and which should I use?
Your current location can be viewed on the GPS in the form of coor-
dinates. Since different maps and charts use different location formats,
Garmin GPS units allow you to choose the correct coordinate system for
the type of map you are using. The most common format is latitude and
longitude, which is utilized by all Garmin units. On most models, you
may choose to change the position format to use with other coordinate
systems. UTM/UPS (Universal Transverse Mercator/Universal Polar
Stereographic) are easy-to-use metric grids that are found on most USGS
topographic quadrangle maps. Several other grids, including a user-defin-
able grid (for the advanced user), may also be selected.
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Appendices
Appendix F: Map Datums and
Position Formats