The "Compass" on my GPS does not point correctly when I am moving slowly or stopped. 
What's Wrong?

What is the difference?

Many people are initially confused by the similarity between the DIRECTION OF TRAVEL indication on a GPS and a COMPASS.  It is easy to be confused when you walk at low speed or stop and look at the indicated DIRECTION OF TRAVEL on your GPS and believe it to be accurate and then it turns out to be WAY off the proper direction for North.

In fact,  the GPS DOES and AT ALL TIMES give you the correct BEARING to your next waypoint. (This assumes good GPS signals and locked and that the coordinates of your next waypoint are accurate.)

What is does NOT give you is a correct pointing arrow toward that bearing at all times.  Why is this?

The GPS computes it's bearing to the next waypoint based on knowing the coordinates of the target AND the GPS receiver's  current location.  (Assuming the target is a km or more away,  the bearing will normally be very accurate indeed. This is true if or not the GPS is in motion or at rest.)   HOWEVER,  the GPS computes it's DIRECTION OF TRAVEL (as opposed to bearing to the next waypoint)  by computing its position at approximately 1 second intervals and calculating DIRECTION OF MOVEMENT.  If you are traveling slowly (for example,  walking at less than about 4 mph or stopped),   measurement errors may cause the GPS to compute that you are moving in some random direction or that you are not moving at all.  In fact,  if you are   moving slowly,  the indicated direction of travel will be some vector addition of   error effects coupled with your actual direction of travel.   If you are STOPPED,  then the indicated direction of travel is the result of apparent motion caused by errors in position measurement caused by  system noise,  atmospheric disturbances,  and other error sources.

When your speed gets up to 4 mph or more,  (more is better),  then you can begin to trust your DIRECTION OF TRAVEL indication on your GPS.  Some of the confusion results because the DIRECTION OF TRAVEL screen on many GPS receivers looks something like a compass.  It is not a compass at all.

In summary:  If you remember that DIRECTION OF TRAVEL and BEARING TO NEXT WAYPOINT are two different measurements you will be OK.  BEARING is always OK on your GPS.   (Walk in the direction your magnetic compass tells you.).  DIRECTION OF TRAVEL as indicated by your GPS can have significant errors if you are traveling at a slow velocity and is totally meaningless if you are stopped..

Joe Mehaffey