Calculating Global Position

GPS Block 1, the first GPS Satellite ever launched, 1978

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Third article in a seven-part series

All maps and charts are produced to a standard that assigns lat/long positions to measured locations and, for the greatest accuracy, you should always change this setting to the one shown on the map you're using. For example, the Garmin series software contains settings for 106 world map datums while the Eagle/Lowrance models contain 192.

How far off could you be using the wrong datum? Let's find out.

Here's my current position. It happens to be where I live and work and the local topographic map indicates the datum to which it is referenced as North American Datum 1927, better known as NAD 27. Some of our regional maps use a later datum called NAD 83.

The NAD 27 position captured by my GPS is:

N 46 19.647'
W 079 22.685'

If I change the map datum in Navigation Setup to NAD 83 the location changes to:

N 46 19.649'
W 079 22.670'

Not much difference there. . . 002' farther north and .015' to the east.

Changing the datum setting to the newer standard, WGS 84, yields a position exactly the same as NAD 83.

But, if I switch to the 1984 Australian Geodetic datum my given position shifts to:

N 46 19.603'
W 079 22.561'

Most topographic maps and marine charts for the continental United States and Canada will be drawn to NAD 27, NAD 83 or WSG 84, standards. But hardy hikers going to other world destinations must be sure to obtain local maps, check the datum, and adjust the GPS before heading into the backcountry.

NOTE:
Your GPS manual details how to change the Map Datum.