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shelee
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Joined: 23 Oct 2002
Posts: 2409
Location: IN
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 Posted: 11/09/02 - 20:02 Post subject: GPS vs. odometer
My husband, the computer, techie gadget dude, who outfits me in just about everything technical he can find (MP3, now a Timex Ironman Triathalon Speed and Distance watch, and has been caught eyeing heart rate monitors...Lord help me..) bought me the aforementioned watch. On my run, I noticed that a route that we had measured with a car was about a half mile longer with the GPS. He thinks there was a "low quality fix off the satellites." How accurate, if anyone else has one, does your GPS watch seem? A 20% error rate is pretty bad. 
Shelly
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copteacher
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Location: Teaching in the Halls of Justice
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Posted: 11/09/02 - 20:45 Post subject:
20% is awful i thought it was closer to 97% acurate
it seems to be that the car could be off too
how old is the car because depending on how long the course is if you odometer is 10% or so off it could make the difference and the GPS is correct
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shelee
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Posted: 11/10/02 - 13:39 Post subject:
| rtpd113 wrote: | 20% is awful i thought it was closer to 97% acurate
it seems to be that the car could be off too
how old is the car because depending on how long the course is if you odometer is 10% or so off it could make the difference and the GPS is correct |
My car is a 2002. My husband thinks that perhaps the GPS was only picking up 2 satellites in the beginning of the route: the first half mile or two miles.
Later in the run (which was just over 9 miles), it found a third satellite and tracked accurately -- he thinks. Most the error was overall was 6%, but 20% in the beginning. I'm not sure why he's putting more faith into our car's odometer than the GPS.
Your right, though, they've been advertised as being extremely accurate. There was a story about a guy measuring distance with a wheel measure and compared results with a GPS watch and error register was less than 1%.
Kind of frustrating. My neighborhood is a mile circumfrence, so I'll lap that a few times around today and see what I get for a reading.
Thanks!
Shelly
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megawill
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Posted: 11/10/02 - 13:58 Post subject:
My experience with the GPS watch is that it is extremely accurate. There are times when running in heavily wooded areas or underneath a highway overpass for example that the signal weakens or may be lost for a couple of seconds. The watch compensates for these instances by assuming that you have traveled in a straight line between the points where the signal is lost to where it is picked up again.
Car odometers are extremely variable and not always accurate. Take your car to a highway that is has mile markers and test your odometer...you can also do this with your watch if you feel comfortable running along side the rode with markers. When I've done this in the past the watch has measured at the very least 97% accurate and 9 times out of 10 it has measured more accurately then that. I'd almost guarantee the the GPS watch is more accurate then 97% of the car odometers out there....(keep that in mind next time you buy a used car)! 
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megawill
Last edited by megawill on 11/10/02 - 14:00; edited 1 time in total
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purple hayes
Frightened Inmate #2
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Location: ON YOUR LEFT!
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Posted: 11/10/02 - 13:59 Post subject:
Try using the GPS in the car and see what the reading difference is. Also, try going to a local track so that you know the distance for 4 laps will be 1600M/1mi.
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shelee
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Posted: 11/11/02 - 19:20 Post subject:
| purple hayes wrote: | | Try using the GPS in the car and see what the reading difference is. Also, try going to a local track so that you know the distance for 4 laps will be 1600M/1mi. |
Megawill, Purplehayes:
Good suggestions! Thanks. Megawill what you said about "catching up" makes perfect sense because when I stopped to wait for traffic the watch may have continued to add milage (I was between giant trees at an intersection maybe losing all satellites) but maybe it was subracted later.
Funny about the car odometer. It seems our 2002 car is off, whereas, our old 89 Trooper is right on. The truck's odometer gave the exact same reading as the watch's odometer. Impressive!
Oh, and I ran two laps around the neighborhood last night and it tracked them right.
Now, wearing the GPS unit is a whole other story. The band is too long for the arm and not quite long enough for the waist. 
Take care,
Shelly
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