Garmin eTrex Legend Cx - Manual (Page 88)

78 eTrex Venture Cx and Legend Cx Owner’s Manual
A
PPENDIX
> M
AP
D
ATUMS AND
L
OCATION
F
ORMATS
Map Datums and Location
Formats
What Are Map Datums?
A datum is a mathematical model of the Earth that
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 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 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, set the
map datum in the GPS unit to the same datum as the
map to ensure the most accurate navigation.
What Is a Location Format?
Your current location is viewed on the GPS in the
form of coordinates. Because 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
used by all Garmin units.
You can choose to change the location format for other
coordinate systems. UTM/UPS (Universal Transverse
Mercator/Universal Polar Stereographic) projection
formats are easy-to-use metric grids that are found on
most USGS topographic quadrangle maps. You can
also select one of several other grids, including a user-
definable grid (for the advanced user).
Map Datums are based on a
mathematical model of the Earth.