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why GPS track distances measure too long

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why GPS track distances measure too long

Postby SSSdave » Fri Aug 26, 2016 8:38 am

The following link with input from a GPS savvy person has the best explanation I've have ever found as to why consumer level GPS devices tend to give inaccurate distances traveled. Frequently on many hiking, highway racing, outdoor sports boards etc people posts their GPS totaled mileage that tends to be greater distances than they are officially known to be.

http://www.sparkpeople.com/mypage_publi ... id=4657789


A fun way to test this at home is to go stand in the middle of your yard or a park and collect multiple points while standing in the spot. Even tho you never move, the points you collect will cover a wide area all around you, sometimes close by, but often many feet away from where you are actually standing...

When you are zoomed out, it looks like it mapped your run, perfectly. However, it tells a different story when you zoom in. Whenever I track my runs on MapMyRun.com, it looks like I was a staggering, drunken trespasser trying to outrun the Physics Police. My line crosses from one side of the street to the other, crossing over and through yards, walls, fences, and houses of people I have never met in my life. If I had actually run this line, I would be battered and bloodied and bruised - and most likely dead, unconscious, or arrested.

Note my only GPS device is on my moto g smartphone. Obviously different consumer level GPS device algorithms may deal with this inherent inaccuracy in different ways or totally ignore such so one cannot simply multiple say a GPS measured distance by a general correction factor and have it come out closer to reality. That inaccuracy may end up worse depending on the settings used like time between readings. And the level of inaccuracy might vary depending on location and terrain. But what one can do for near areas is apply a correction factor on a given trail based on a known relative accurate distance.

Lets say the generally accepted trail distance between Trailhead A and Lake B is 2.4 miles. If one's GPS device read 3.0 miles then the correction factor is 2.4/3.0= 0.8. Accordingly if one continued hiking in that area over several days, applying the 0.8 correction factor to all readings would likely improve actual distances. But later if one applied the same correction factor to the same GPS device tracking distances traveled while driving one's car along a mountain highway, the result might be quite different and one may find the inaccuracy was really only 0.95 say.

Some highways have accurate mileage markers one can easily calibrate their car odometer to. Most vehicles if using their stock tires show rather accurate readings but if not that will show such. In the same way sans trail, a person could calibrate their foot hiking by say first calibrating their vehicle odometer so and then say drive around a quiet residential block for a distance of about a mile. Then one could walk on the road around the block and see how that compares. In this case I used a loop instead of a straight distance along a road in case there were variables with distance and making turns. And if one's GPS device seemed to consistently have a similar correction factor regardless of location etc then one may be able to use that generally on readings.

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Re: why GPS track distances measure too long

Postby AlmostThere » Fri Aug 26, 2016 9:00 am

A friend uses a pedometer. A route of a few miles through talus ran to nine miles in actuality.... Trail walking is a little different, true, but another variable might be also those variances, from inches to feet, in how high one must lift the leg, those minor detours around things like washouts or heading up the stream to cross on rocks instead of wading, that all add up a lot.

I experimented for a while with lengthening my stride by only a few inches as I hiked. Over miles it makes a BIG difference.

As usual in these matters, it depends.
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Re: why GPS track distances measure too long

Postby psykokid » Fri Aug 26, 2016 9:20 am

Sounds legit. I have recorded tracks on my first gen Moto X with Strava and with my old Garmin etrex 20 concurrently. The etrex pulls location from the regular GPS sats as well as the russian Glonass sats so it's a bit more precise. I've got an out and back hike I do 3x a week and it's 5.4 miles round trip. My phone is always off by around plus two tenths of a mile. My Garmin is spot on.
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Re: why GPS track distances measure too long

Postby seanr » Fri Aug 26, 2016 9:57 am

Interesting topic as it frequently has come up on long hikes I've experienced with GPS carrying folks that the mileage indicated on the devices indicated us traveling substantially farther than anticipated. Sometimes it has to do with rugged, cross country travel, but now I see there may be much more to the story at times.

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