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Reference
Map Datums
Eighteen position
formats are available in
the eTrex.
Data Entry Pages exist
for the User Grid Format
and the User Datum
Setup. These pages allow
you to configure the eTrex
for compatibility with
custom designed grids. We
recommend surfing the
net for links to Garmin
user-grid and user datum
information. There are
also excellent navigation
and GPS articles
available on-line which
will enhance your overall
knowledge of GPS.
32
Position Format
The default position format (and the one most commonly
used) for the eTrex is latitude and longitude in degrees and
minutes (hdddº mm.mmm'). You may also choose from 17
other position formats. There is also a "User" position for-
mat which allows you to approximate custom-designed
grids. For more information on grids and datum we rec-
ommend you visit the National Imagery and Mapping
Agency at http://www.nima.mil.
To change the Position Format (grid):
1. Press the PAGE Button and switch to the 'MENU'
Page. Press the UP or DOWN Button and highlight
'SETUP'. Press ENTER. Use the UP/DOWN buttons
to select 'UNITS'. Press ENTER. The 'UNITS' Page
appears.
2. Press the UP or DOWN Button and select the
'POSITION FRMT' field. Press ENTER.
3. Press the UP or DOWN Button and select the
desired format. Press ENTER. Press PAGE to get
back to the page of your choice.
Map Datums
A map datum is a reference model of the earth' s
shape. Every map uses a map datum. The map datum is
usually listed in the title block of the map. If you are
comparing GPS coordinates to a paper map or other
reference, the map datum in the eTrex should match the
horizontal map datum used to generate the chart or other
reference.
The default datum in the eTrex is WGS 84 (World
Geodetic Survey, 1984). You should only change the
datum if you are using maps or charts that specify a
different datum than WGS 84. If no datum is specified,
you can select each datum applicable to your region until
you find the datum that provides the best positioning at a
known point. You may also choose the "User" datum for
use with custom-designed grids.
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