Your "measured" position by a GPS receiver is never better than a 10' margin of error in ideal conditions in North America, so when you measure distance with a GPS, perfect conditions will yield a 20' margin of error. And conditions are rarely ideal. If there were trees, buildings, atmospheric interference, or any other kind of error introduced, you could be 100' off by the GPS measurement and not realize it. The accuracy of your calculation could also be harmed by the significant digits in the GPS's measured distance, since you're converting from kilometers to feet.