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For additional information on Loran TD setup, see page
78
.
• Map Datum – allows you to manually select the datum reference used to determine a given position.
The default setting is ‘WGS 84’. The unit automatically chooses the best datum depending upon
your chosen position format. Datums are used to describe geographic positions for surveying,
mapping, and navigation and are not actual maps built in the unit. Although over 100 datums are
available (as listed on pages
83-84
), 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.
• Heading – lets you select 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 which are automatically determined from your current position. ‘True’ provides headings
based upon 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 position 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 ARROW KEYPAD to enter the magnetic variation at your current position and press
ENTER.
• Distance, Speed and Elevation – lets you select the desired units of measure for distance, speed and
elevation readouts in ‘Nautical’ (nm, kt, ft), ‘Nautical’ (nm, kt, m), ‘Statute’ (mi, mh, ft) or ‘Metric’
(km, kh, m) terms.
• Depth – lets you select the desired units of measure for depth in Feet (ft), Fathoms (fa) or Meters (m).
• Temperature – lets you select the desired units of measure for temperature in Fahrenheit (°F) or
Celsius (°C).
Main Menu: Units Tab
WARNING: If ‘User Mag Var’ is selected, you must
periodically update the magnetic variation as your position
changes. Using this setting, the unit does not automati-
cally calculate and update the magnetic variation at your
present position. Failure to update this setting may result in
substantial differences between the information displayed
on your unit and external references, such as a magnetic
compass.
The Units tab settings affect how navigation information is
displayed on the GPSMAP 172/172C. When in doubt, seek
assistance.
If you are using a map or chart in conjunction with your
GPS, make sure the settings under the ‘Units’ tab of the
GPSMAP 172/172C match that of the map or chart. This
information should be displayed somewhere on the map or
chart. If the information is not present, contact the map or
chart maker to determine what position format, datum,
heading and units of measure are being used for the map
or chart.
71
Reference