Why am I lost when I have a GPS?

Ever wondered how you could possibly get lost while carrying a GPS, and when you do get geographically embarrassed, whether you can trust your GPS?  


I've sometimes been acutely embarrassed when my high-tech equipment has been worse than useless on a difficult walk where spot-on navigation was essential. More than once,  I've found myself on the opposite side of a creek or on the wrong ridge-line. Sometimes I've returned from a walk to find that my route lay parallel to that shown on the map, but about 200m away. How could my GPS get it so wrong?


Well of course my GPS hasn't got it wrong, it has been simply calculating position based on the incorrect settings I gave it. The indication that you've incorrectly set your GPS is when every feature is in the wrong place by a consistent amount.


If the error is about 200m then you have probably set your GPS to the wrong map datum eg you may have chosen Aus Geod '84 or Aus Geod '66 when your map shows the correct datum to be GDA94 . Your GPS must be set to the same map datum as  the map you are using.

For my Garmin Geko 201, the following datums are relevant when using Australian maps.
    Aus Geod  '84 = AGD84
    Aus Geod '66 = AGD66
    GDA  =GDA94
For Australian maps, the common map datums you will find are
    AGD66 (oldest)
    AGD84
    GDA94 (most recent)


Fortunately, all you have to do is to make sure that the map datum to which you set your GPS matches the one shown in the legend on your map. To do this for a Garmin Geko, select SETUP from the menu, then UNITS from the sub-menu, then MAP DATUM, and scroll down the list to select the correct one which matches your map.

References:  

Sydney Bushwalkers Navigation Resources
Map datum for GPS newbies
Geoscience Australia 

This article by Bush Walker is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.