Garmin GPSMAP 172C Owner's Manual - Page 81
Main Menu: Units Tab, To enter a user defined magnetic variation
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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. I 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). Reference Main Menu: Units Tab I 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. I 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 automatically 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. 71