OK Bob, I'm asking more questions.
I have a GPS unit I have used on the ground. I have also worked with a surveying crew
using the latest GPS equipment, backpack etc like you describe. To eliminate the random
drifting the military built into the system the professional equipment uses a base
station set up on a point that doesn't move. The remotes are in radio contact with the
base and since the base knows it doesn't move the remotes can account for the drifting
by the information the base sends out. If the remotes lose contact with the base they
won't give you any data. (At least the unit we used which was being demoed by the field
surveyors for the company trying to sell us the unit.)
So I don't see how a hand held can account for the drifting. And since I can stand
still and watch the handheld tell me I'm moving I know there would be error in every
point it grabs. Since we agree the error can be around 100 meters I'm afraid the error
in measuring a course could be large.
I also did a little analysis about measuring a curved trail. Assuming a curve at a
radius of 500 feet and traveling at 8 mph and points being grabbed every 15 seconds, the
straight line error would be .75%. Not much and I could live with that. But then I
think, if it is grabbing a point every 15 seconds which is 175 feet traveled at 8mph.
And the drifting error can be 300 feet, wow, we could have some big errors! I don't know
a lot about the speed of the military induced drift and maybe it only drifts 50 feet in
15 seconds but this could still cause huge errors.
Personally I still like mountain bikes, motorcycles or surveyors wheels with
calibrated and checked odometers.
Jim Mitchell
Bakersfield, Ca.