The best argument I have read to date regarding the differences in measurements using GPS and the Jones Counter is the testing results Stuart Gordon had published here in January, see http://www.cage.curtin.edu.au/~gordonsj/course/.
Stuart’s results simply takes the air out of those suggesting that they’re newly purchased GPS wrist watch can determine the “true” length of the road course they just ran. New technologies really can be a little dangerous in the hands of a few.
It will be further interesting reading when Stuart publishes his results on aerial photogrammetry.