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eTrex Vista C
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
eTrex Vista C 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 ensure the most accurate
navigation.
What is a Position Format, and which should I use?
Your current location can be viewed on the GPS in the form of
coordinates. 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-defi nable grid (for the advanced user), may also be selected.
Map Datums are based on a
mathematical model of the Earth.
Position
Formats
Map Datums
Map Datums and Position Formats
Appendix
eTrex Vista C Owner's Manual Text.indd 88 8/12/2004, 9:18:37 AM