Good observation Mark!
I have only heard a “short course” GPS concern expressed one time. Every other challenge was due to the GPS “seeing” a long course.
The one short was easy to explain. That 10K (NY09062JG) has a lot of tree cover forming a thick canopy over much of the course plus there are signal blocking ravines. Ron is correct. The GPS unit must assume a straight line had been traveled during the outage when the signal is reacquired and the GPS wearer is further down the road.
While measuring that 10K my GPS once reported a sudden elevation jump of 100 feet! Not in reality of coarse, just on the GPS screen. That’s very disappointing. I was hoping the GPS was magic.
Sometimes the GPS wearers are helpful. I was alarmed after a race on one of my 5Ks earlier this year. Many GPS wearers reported 1/10 mile long course! Some were mad! Signal reception should be excellent over that entire route. Fearing the worst, I rushed out to see what might have gone wrong with my measurement.
Nothing. The course (NY11029JG) was still well marked and not physically altered. Everything checked out fine. The map couldn’t have been clearer. But the GPS should not be THAT wrong with all this open sky and a small race such that the GPS runners should not have been that far off the shortest possible route.
Fortunately, the event had captured promotional video of the race. We studied it… It clearly showed that the race had permitted a sub-contractor (the timer) to inexplicably set up the finish line exactly 1/10 mile further down the road. The GPS wearers were RIGHT! The course they ran was 1/10th mile too long.
I was glad I had held my tongue and not given the "GPS Geeks" my technology lecture before learning what actually had happened...
JJ