Originally Posted by
njkayaker
There is nothing about cul-de-sacs that would make GPS measurements less accurate.
Unfortunately not true.
It is update rate. the GPS only records a position every so many seconds (or fractions of one). It computes distance traveled by taking the distance between the two waypoints and dividing by time to get speed. If the GPS records one waypoint at the entrance to the circle and one at the end, well it doesn't take a math genius to figure out it won't compute an accurate distance.
The Garmin I have varies how often it records the waypoints saving space on the memory card. it also has an option to switch to a fixed, fairly high record rate. Recording the waypoints more frequently improves the accuracy of the calculations.