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:  

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

3 thoughts on “Why am I lost when I have a GPS?”

  1. Thanks for the info.. I just got my GPS and exactly that happened.. will set it up properly for the next walk.. glad I didn't go cross country but a well signed track so couldn't get lost..hehe.

    Like

  2. That makes it too easy I wish the bloke at the store had put more effort into making sure my garmin was set up corectly in stead of spinning bullcrap of how good the unit was and buy this and get that just put the sd card in and off u go he rekons I got lost for 4 hours before I went back to the good old paper map and got myself out not a good feeling when you have spent all that coin! So thankyou mate for making a useless paperweight into an invaluable piece of my bushwalking toolery cheers

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s