GPS route different distance than Google map route???????????

r_runner

posted: 11/20/2007 at 11:43 AM

OK, this is really bugging me. I was curious to see how accurate the distance was for the google maps route I created. So, I dug out my Garmin Rino 120 and walked to the halfway point. The GPS read a distance of 1.35 miles while the google satellite route read a distance of 1.44 miles. I created plenty of segments to represent the route accurately, so what gives? I don't think I lost signal anywhere along the trail. I guess I'll have to download the route to make sure nothing is funky, but can anybody explain the discrepeancy. I feel there is something I'm missing... -Ryan

Berner

posted: 11/20/2007 at 12:14 PM
modified: 11/20/2007 at 12:15 PM

What kind of accuracy reading was your GPS device giving (if it gives you such information)? Its possible that you didn't have a good signal lock. Also, you're looking at a distance discrepancy of .09 miles, or roughly 6% of the distance. That's a bit high for today's consumer GPS devices, but not outside the range of reasonableness. Between inherent online mapping discrepancies and inherent GPS inaccuracies -- coupled with the short distance being measured -- I don't know that the difference is necessarily meaningful.

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mbk

posted: 11/20/2007 at 12:52 PM
modified: 11/20/2007 at 12:53 PM

I run the same short list of routes almost every day. They never match each other or the Google route I've drawn. They are close, but always off, sometimes by .1 or .2 miles. I think GPS is a great technology but it is limited and there are always small discrepancies. Also, how you round a corner, hills, any weaving, etc. will make the distances off. I've noticed that straight paths are more accurate than curved paths or paths with frequent turns. There have been some other posts about the accuracy of GPS devices, and I think some of them got pretty technical, so you might find some good information if you search the forums. Of course just about every third post has Garmin or GPS in it, so it may be a challenge to find the posts of which I speak! Wink

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Blaine Moore (MM#2867)

posted: 11/20/2007 at 2:28 PM

Another factor is elevation change. Google mashups don't take hills into account when figuring out the distance, and the GPS technologies are better but not 100% accurate at telling you the correct elevation. That can account for some of the differences.

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greenrunning

posted: 1/12/2008 at 10:41 PM

I also noticed on my 305 that the "5 mile" route I always run near my house and mapped with mapmyrun.com (google maps) registers as 4.9. I guess it's not a big deal, but it I'm pretty anal about my time/distance, and now I have to decide do I run the extra .1, which throws off my time/pace a little, or do i consider this route to be 5 miles and not go by my 305's time, which will make my RA log not match my sporttrack log. Yes I know it's only .1, but it throws everything out of whack. I know I'm not the only one who keeps multiple logs...do you guys just go by what your GPS says even if you know it's slightly inaccurate? I'm assuming that other routes I haven't tried with the 305 yet could be off by even more.

Trent

posted: 1/12/2008 at 10:52 PM

Quote from Run To Win on 11/20/2007 at 2:28 PM:

Another factor is elevation change. Google mashups don't take hills into account when figuring out the distance, and the GPS technologies are better but not 100% accurate at telling you the correct elevation. That can account for some of the differences.

The effect of elevation change on distance is minimal, usually far less than the innate error of GPS measurements and the measurements for point and click errors with gmaps. This was just brought up on another thread. Check it.

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PWL

posted: 1/13/2008 at 10:36 AM

Quote from greenrunning on 1/12/2008 at 10:41 PM:

...do you guys just go by what your GPS says even if you know it's slightly inaccurate?

I just go by what my 305 says--it is generally accurate within about 1-2% of what Google says. In the end, that is close enough for me as a "recreational" runner. Besides, who knows if Google is any more accurate anyway? I knew one guy who had an Excel spreadsheet that averaged the distances from his Garmin, from Google maps, and from his Polar footpod. Seems a bit over-the-top to me, but to each his own I guess. Wink

"I would never die for my beliefs, because I might be wrong."--Bertrand Russell

Al1

posted: 1/13/2008 at 10:56 AM

I have found that the more I zoom in when making a google map course, the closer it will match my Garmin 301. Today, I ran a new course that register 6.35 miles on my Garmin and 6.37 miles by google maps. When I first got my garmin, I went to the track to check it out. The Garmin was way off doing that. It wanted to run straight lines in the corners. So, now if I run splits at the track, I turn off the GPS and just use the stopwatch.

GPS route different distance than Google map route??????????? (Read 1716 times)