GPS units measure horizontal distance, they do not
measure the slope component. So going up and down that canyon Angie mentions
will measure short on the GPS. I have crossed checked my GPS with wheel
measurements, accurate wheel measurements, and found the GPS to be short in all
up and down or winding trail situations. On fairly straight roads, in a car,
they do pretty well, but not typical endurance trails out here.
As far as using a GPS to measure
an endurance trail, Julie Suhr always says, a horse and rider need to try the
whole trail, before the ride, and make sure it can be done in one day in the
tine allotted. That will find many of your mileage and trail footing errors
before all the people come to the ride.
