Can anybody explain this ...............

DNEchris

Barefooters
Jun 25, 2010
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Bronxville NY
I went out for a run today wearing my regular Polar RCX5 and a TomTom runner. Both are GPS equipped. At the end of the run I downloaded both and they had recorded the same operational time but the TomTom showed 0.4 miles shorter distance than the 14.2 recorded by the Polar. I plotted my route on Google maps and that distance agreed with the Polar.
Does anybody have any ideas as to what could have led to the discrepancy?
 
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For what it's worth: I still run with a Garmin Forerunner 110. When it was new, there were only slight discrepancies for the same run on different days. By now the batteries are getting weak (with Garmin you can't change them and have to buy a new watch instead :mad:), time is much longer to find a satellite - and, what is even more annoying, it seems to lose the satellite from time to time, which means that at the end of the run it shows one or two kilometers less than for the same run two years ago.
 
I read Sid's first link but not the rest (there looked to be several there). Was it mentioned about the issues next to tall buildings with reflected signals? Not sure if you run by tall buildings there Chris, but since in NYC I figure that might be a possibility. I've noticed this a lot when I run races in downtown Columbus. When next to a skyscraper I've heard the satellite signals can be reflected off the walls. GPS works on timing how long the signal takes between the satellites and your receiver, so a signal that bounces off a building will be slightly longer than one that is direct, so your gps thinks your further away. This can clearly be seen when you review your track on a map... as I go by a large building my path curves away from it, sometimes nearly to the next road over. This will add mileage, and make you appear to be traveling at a faster rate.

However with a good view of the sky, no buildings, my really old (6-8yrs?) Garmin 205 does great. Much better than my new smartphone (purchased this spring) running endomondo. That thing I'm not sure what ails it. Sometimes its ok, sometimes it drops signal constantly, and I've been off as far as 2 miles on 6-8mi runs a couple times. Not sure there if its endomondo, my receiver, or a combination of the two. I suspect it to be a combination between a poor receiver and software that doesn't handle that very well. When out on the open highway using it in my vehicle it does pretty well - and I use it for a speedometer since mine is off in my truck.

A couple examples of crappy performance using my phone... I'm running in about a perfect straight line.

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Not sure if you run by tall buildings there Chris,
There are one or two ;)

The intriguing thing was that both units recorded the same route. I've certainly had issues in the past with the Polar where it would appear that I've covered a couple of miles at about twice the speed of light - but there were no such aberrations yesterday.