There are a lot of variables in how accurate a GPS is going to be.
Many of the units will measrue the time between the current fix and the
last good fix. GPS also transfers time - so it knows time from the
satellite of the fix. If this time period is too long, it will reset
and must likely (especially the units in the couple hundred dollar and
down range ) won't tell you.
I did a ride in WY a few years. This was mostly in the open but a lot
of shadowing from valleys and canyons. The first look was 25 miles so
for grins and giggles I took my GPS unit. It measured the 25 mile loop
(which I am sure was 25 miles) at 11!
Also speed can make a difference. If you are walking through the woods
- althoug the fixes will be sporatic - they most likely will not fall
outside the time window for resetting. If you are trotting or galloping
they quite frequently can. I have a 12 mile loop ( and I know it is 12
miles ) that I regularly ride. When I walk it my GPS is fairly accurate
- maybe short by 5 to10 percent. If I trot the whole thing there
reading will be all over the map from 3 miles to 8 miles.
It's a great tool for what it was designed to do - provide navigation
and time transfer. But for accurately and precisely measuring mountain
trails, canyon trails, forest trails, etc. - there are too many
variables.
BTW, a GPS is accurate to a few feet in determining a single position.
It isn't that good at determining the length of a squiggly path. I don't
think they compensate for elevation change, and I know that when it
can't read, it just assumes a straight line. John Teeter told me that he
figured a GPS measured about 85-90% of actual distance, and where he
lives tree cover isn't a problem. One ride I went to (name withheld
because it was an otherwise nice ride) where the distances between vet
checks were most screwed up was measured with a GPS and had considerable
forest.
That's why I don't like or trust GPS as a mileage-measuring tool. It is a
great thing to "find yourself" and keep track of where you are, but give
me an odometer any time for the actual mileage. It isn't that tough to
check one's odometer so that one can at least adjust for its inaccuracies.
And most rides can be measured that way to a large extent. A mileage
wheel also works for the areas where one can't get with an odometer.
