I hope someone can explain when measuring with the Jones counter on a hilly course your GPS reading are not in sync as Keith Lively suggests. Would the GPS give a long course or short course(if the course is hilly)?

Thanks,
Gene
Original Post

GPS for course measurement is not accurate enough to give a statement on whether it would show a long or a short course. Some folks in Australia did a comparison between Differential GPS (DGPS), which is much more accurate than normal GPS, and the Jones Counter. Even with DGPS, the accuracy was not very good. For example, when the course went under trees, things got really bad.
I've done quite a bit of playing around with my three different GPS units. I have found that if you are completely in the open (and don't forget your body, even your hand, will shield the GPS signal) you get a reasonably accurate distance. On my usual 10km run which is mostly open I get between 9.9 and 10.1. If there are trees or the signal is weak for some reason the accuracy falls off. With a weak signal the GPS position varies (perhaps 1st to the left of your location and then to the right) so the GPS thinks it has gone farther than it has - it will read long. On the other hand if you loose the signal for a period and then it is re-established the GPS assumes a straight line so it will read short if you made many turns while the signal was lost.
GPS will always assume a flat (although not necessary level) distance from one point to another. Therefore if there is a hill between points, the GPS will "flatten" the distance, and the actual distance over the ground will be greater than the GPS distance.

GPS accuracy is quite dependent on the signal quality. Newer GPS systems use WAAS (Wide Area Augmentation System)technology which is accurate to 2 - 3 meters horizontally. This accuracy factor is is not clumulative - so over 5k one might have a 2 - 3 meter error, and over 26 miles the error would also be 2 - 3 meters, but the distance would be from one point in space to another, without regard for distance traveled over ground. (Remember that GPS was developed primarily for aeronautical purposes.)

A good GPS would certainly be a useful tool for laying out a course, to be finally measured by established practice.

Dick Blum
ryblum@mchsi.com

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