Here's a real world test for you without having to program anything.
Note: with a little knowledge of geometry, this first bit could be done on paper. But, just in case...
With your
boat in the slip, the stern tied tight so it can't move side to side:
Move the front side to side while someone looks at your
compass. Measure how many feet/inches the front moves for 1 degree of
compass movement.
Now, take your
GPS. Put it in a mode where you can see the exact position displayed. See how much the numbers change when it is moved the distance required to measure 1 degree on your
boat. You'll need to test at various angles. I'd think the worst performance would be aimed 45 degrees off.
You may have to actually connect the
GPS to the computer and use some
software (like OpenCPN) as the display. This is because the GPS may not display all if its decimal places. Someone in here that has programmed
OpenCPN or Gippy might have an idea of how many decimal places are reported by common
GPS receivers.
-dan