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Good article. It could be used in running club newsletters.
I still don't like using the term "added distance" to describe the SCPF. This makes the uninitiated runner think that a certified 5K is actually 5 meters long; it is not. The SCPF doesn't really add distance to a perfect 5K, it compensates for the fact that a human can't ride a perfectly straight line on a bicycle.
The original running Garmin was a GPS device you strapped on your arm and sent its info to a separate watch that you wore. I had one of these and it was very inconsistent at measuring distance. We bought one of the first watch-type devices, and that's what I've used for my testing along with my hand-held device.
A few years ago I asked people in a road race to report their GPS measurement and their model #. From those reports I didn't see any significant difference between the different models, older and newer. But in that race the view of the sky was mostly unobstructed. I suspect the improvements with the newer watches is their ability to hold onto a signal in less than ideal conditions. However, with my very old watch the only time I've had signal problems is in locations with tall buildings. But maybe with the newer watches the difference is the definition of "tall."
The posted article is actually quite old and has been posted on the bulletin board before. It is a bit surprising to me that someone who has access to multiple GPS devices wouldn't have tested some of them to see how accurate they actually are at measuring distance before writing that article. Other than Bob Baumel's tests of his iPhone (which we all agree now is not an accurate GPS device for measuring distance), I haven't seen anyone report tests of a GPS device measuring distance that have not shown them to be accurate.
I don't have a "brand new" Garmin, but I started using GPS technology back in the 90's using a hand-held unit meant for hiking. It was great for mile marker spotting. Once the civil accuracy was improved (sometime in the 90's) finding a specific spot with them was very good.

That said, I've found them very environmentally sensitive. I think this is why some people will say a model is spot on, and another will not. I think this accounts for the "it reads for me all the time so you have to be wrong" scenario also. As I've mentioned before in the forum I used to get water as a country club near here, and the GPS track would have me zooming down the road in the opposite direction and then recovering a 1/4 mile later. Other have reported the same affect there, so there's a funky reflection going on. Going around the track near my house in the summer results in tracks that resemble a drunken sailor.

While some models may be more sensitive than others, any study should look more at the conditions where unreliable readings are likely, not just the models that aren't affected as much.

People will always say "well the military can track you within a 1/2 inch", but that's not a wrist watch, and they're not measuring distance traveled. It's an entirely different matter to measure how to get to spot x with a certain percision than it is to measure the path along a series of those spots.

What I tell people when they challenge a distance based on GPS (when I'm in a good mood) is that if the course was laid out per the map, and if the procedures for measurement were done right, the process yields an extremely accurate course and knowing all the things that can affect the measured distance on a GPS there are simply way more reasons to suspect a GPS measurement than a bicycle one.

I've done numerous head to head comparisons of Jones Counter vs Garmin 610 because I always have it on the bike when I measure. I use the tracks to speed the generation of the map. Most of the time it is very close, but it's wrong often enough to not trust the accuracy as much as many do. In addition to tall buildings, trees and cliffs are problematic, but I think there are other factors I haven't identified since the incidents by the local country club seem more than a tree could cause.
GPS compares the time it takes radio waves to travel from the satellite to the receiver. There are a huge number of factors that will cause fluctuations from one measurement to the next.

Since they are radio waves, they can be reflected off any of a number of objects. Power lines and metal fences can be some distance away and still have an impact. Anything remotely reflective can cause a bad point.

There are many atmospheric factors that will make an impact as well. From density in the troposphere, to solar storms in the ionosphere, air plays a large part.

Civilian GPS isn't supposed to be as accurate as possible. Accuracy is improving as more channels are added, and the network of satellites is upgraded from time-to-time.

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