Filter
Top Products
GPSMAP 182/182C Owner’s Manual 71
MAIN MENU > UNITS TAB
• Map Datum—sets the datum reference used to determine
a given location. The default setting is WGS 84. The
unit automatically chooses the best datum depending on
your chosen position format. Datums are used to describe
geographic locations for surveying, mapping, and navigation.
You should only change the datum if you are using a map or
chart that specifies a different datum.
WARNING: Selecting the wrong map datum can result in
substantial position errors. Unless otherwise specified by
the navigation documentation you are using, use the default
WGS 84 datum for best overall performance.
• Heading—selects the reference used in calculating heading
information. You can select from Auto Mag Var, True, Grid,
and User Mag Var. Auto Mag Var provides magnetic north
heading references that are automatically determined from
your current location. True provides headings based on a
true north reference. Grid provides headings based on a grid
north reference (and is used in conjunction with the grid
position formats described on page 70). User Mag Var allows
you to specify the magnetic variation at your current location
and provides magnetic north heading references based upon
the variation you enter.
To enter a user defined
magnetic variation:
1. With the Heading field set to User Mag Var, highlight the
heading reference field (immediately to the right), and press
ENTER.
2. Use the ROCKER to enter the magnetic variation at your
current location, and press ENTER.
WARNING: If User Mag Var is selected, you must periodically
update the magnetic variation as your location changes. Using
this setting, the unit does not automatically calculate and
update the magnetic variation at your present location. Failure
to update this setting can result in substantial differences
between the information displayed on your unit and external
references, such as a magnetic compass.