Map Datums; Position Format - Garmin eTrex Owner's Manual

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
confi gure the eTrex for
compatibility with custom
designed grids. We
recommend surfi ng 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.
34

Position Format

The default position coordinates format (and the one
most commonly used) for the eTrex is latitude and lon-
gitude in degrees and minutes (hdddº mm.mmm'). You
may also choose from 17 other position formats. There is
also a "User" position format which allows you to approx-
imate custom-designed grids. For more information on
grids and datums we recommend 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 or DOWN
Button to select 'UNITS'. Press ENTER. The 'UNITS'
Page appears.
2. Press the UP or DOWN Button and select the
'POSITION FRMT' fi eld. 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 specifi ed,
you can select each datum applicable to your region until
you fi nd the datum that provides the best positioning at a
known point. You may also choose the "User" datum for
use with custom-designed grids.