measure up four inches and to the left a
To use this format, you need to follow these steps in order. First, take your map of the area and determine a reference latitude/longitude. (Note: in order for this system to work, the latitude/longitude lines must be parallel with the edge of the map. USGS maps are parallel, others may not be. Also, this works better with smaller scale maps.) The reference position can be anywhere on the map, but the closer it is to your location, the smaller the numbers will be that you’ll have to deal with.
Once you’ve decided on a reference position, you can save it as a waypoint. See the waypoint section for information on saving a waypoint. Save the reference position as a waypoint. Exit from the waypoint screens.
Now select “Configure Map Fix” from the “Coordinate System” menu. The screen at right ap- pears. “Map Scale” is highlighted. Press the right arrow key. Enter the map’s scale. This is generally at the bottom of the paper map. It’s shown as a ratio, for example 1:24000. The unit returns to the Configure Map Fix screen. Now select “Select Origin Waypoint” if you saved the reference point as a waypoint or highlight the Lati-
tude or Longitude under the “Origin” and enter the reference position. If you used “Select Origin Waypoint”, then select the waypoint that you saved the reference point under. The unit returns to the Configure Map Fix menu.
Finally, press the EXIT key to erase this menu. Now select "Map Fix” from the list and press the ENT key. All position information now shows as a distance from the reference point you chose.
Datum
Maps and charts are based on a survey of the area that’s covered by the map or chart. These surveys are called “Datums”. Maps that are created using different datums will show the same latitude/longitude in slightly different locations.
All datums are named. The GPS system is based on the
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