Am_Fear_Liath_Mor,

Thanks for illustrating the various sources of errors in GPS navigation.

I have one little caveat: The GPS simply isn't used the way you describe. The electronic compass is just a useless battery hog. The best, easiest and most intuitive way to use a GPS is to use waypoints and see how your position is displaced on the GPS screen in relaton to those waypoints.

Tell your GPS "I want to go to that waypoint (or place your marker on the map)" and you have lots of fancy graphics showing you if you are dead on or need to veer left or right. No need to ever use the compass, neither electronic nor conventional (but you need the conventional as a backup).

So the most relevant inaccuracy is the difference between the different datums (nad27 versus wgs-84). In your example you've used 200 meters. In my neck of the woods, the difference can be as much as 400 meters between ED50 (European datum, 1950, still used on many nautical maps and some really old topo maps) and WGS-84. And - that difference is only relevant when you do translate coordinates from your paper map to your GPS, or vice versa. The GPS map display will show your correct position, no matter what datum the GPS is set to. (And yes, even the best GPS map display is totally inferior to a good topo map, which gives you both the fine details and a large overview without a need to zoom in or out).


200 or 400 meters may be close to insignificant or really make you doubt both the GPS and your mapping skills - it depends of what kind of features that is between your real and your fake position. It could send you down the wrong trail, into the wrong water shed (valley) or put you on the wrong side of a river or a road. But a glimpse on the gps map display would set you right.

There is also an inaccuracy when dealing with the coordinate grid of a topo map. On a 1:50.000 map with a one kilometer UTM grid you are really good if you can do that with 100 meter accuracy (plus/minus 50 meters, or one millimeter on that scale). A user inaccuracy of 400 meters when tired, under stress and so on is entirely plausible.