Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Monday, September 17, 2012

The Compass in Your GPS

Is your GPS set up correctly?  Does your GPS's electronic compass match with your magnetic compass?

The hunter is at the trail head ready to strike out for the back country.  The GPS receiver is powered on and is tracking 10 satellites.  The receiver’s batteries were replaced earlier at camp.  All looks good. 
The hunter marks a waypoint to record the location of trail head and truck. 

While moving down the trail, the hunter notices that the GPS receiver’s electronic compass just “doesn’t seem to look right.”  He checks his magnetic compass and confirms his direction of travel with the topographic map.  The GPS receiver's compass is about 15° off.  Not far from the trail head and with the hunter’s truck in view, he takes a bearing and neither magnetic compass nor the electronic compass match.

There are two considerations that need to take into account.

First, every time the GPS receiver’s batteries are replaced, the electronic compass needs to be calibrated.  It’s a simple process that requires a quick check of the owner’s manual.

Second, both the compass and GPS receiver must be set to complement each other.  For example, if the hunter has a basic base plate compass (one that cannot be adjusted for declination)  then the GPS receiver’s “north reference” must be set to magnetic.  If the hunter has a compass adjusted for declination then the receiver should be set to true north.  If compass and GPS receiver don’t match then the bearing information may be as much as 10° to 20° off.  That is not good.

I carry a Sylva Ranger style compass that can be adjusted for declination.  Before leaving home I visit to verify the correct declination for my planned hunt location.  With that information I adjust the compass.  Yes, the magnetic needle still points to magnetic north but the rotating dial provides degree/azimuth information in degrees true.  I’ll then set my GPS receiver’s “north reference” to true north.

Now my GPS and compass settings match my topographic map. That is the best way to do it.

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