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
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