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    GPS, you've gone too far this time

    This is a discussion on GPS, you've gone too far this time within the Everyday Life forums, part of the Community channel category; GPS, you've gone too far this time Built-in bias means your walk tracker over-estimates distances People writing GPS software need ...

    1. #1
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      GPS, you've gone too far this time

      GPS, you've gone too far this time
      Built-in bias means your walk tracker over-estimates distances


      People writing GPS software need to re-think their approach to measuring distance, because the system has a built-in bias to over-estimate how far you've moved.

      That's the conclusion of a group led by geo-boffin Peter Ranacher of the University of Salzburg, in a paper published in the International Journal of Geographical Information Science.

      If errors were merely random, you'd expect under- and over-estimating measurements to be equally distributed, but that's not what happens according to Ranacher's group. Instead, if geographic points (latitude and longitude) are used to calculate distance, it always results in an over-estimate.*

      The too-long-didn't-read version is in the abstract, where they write: “GPS movement data are affected by both measurement and interpolation errors. In this article we show that measurement error causes a systematic bias in distances recorded with a GPS; the distance between two points recorded with a GPS is – on average – bigger than the true distance between these points”.

      After an awful lot of mathematics that explains the statistical reasons for the over-estimation, the researchers tested the idea by staking out a 10 metre square and undertaking the joyous task of walking around it 25 times, recording their position when passing reference markers every meter.

      In a second, less-tedious experiment, the boffins measured the movements of a car with a GPS logger for six days, ending up with around 195 km of the car's trajectories to work on.

      For applications where accurate distance measurements are required, the researchers suggest, GPS software should use speed information instead of trying to measure the distance between points: “point speed measurements are calculated from the instantaneous derivative of the GPS signal using the Doppler effect”, they point out, and are therefore highly accurate.

      Point speed estimates at high sampling rates would, they write, provide a self-correction mechanism that would correct errors in GPS position estimates, without having to test individual systems to get “ground truth” data


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    3. #2
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      Re: GPS, you've gone too far this time

      Since you've clearly read the full report in great detail and checked the calculations, how many centimetres out was the car after 195 km?

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      Re: GPS, you've gone too far this time

      How about in the wrong town?

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      Re: GPS, you've gone too far this time

      I have just read the paper. What I understand it to say is that GPS measurements are inaccurate spatially when the movement is less than 5 metres. (And the error can be several metres.) At greater distances the error margin is smaller. I wonder which GPS system they were using, since the system accuracy for civilian use is + or - 7.8 metres. If that is so, a recorded error of 5 metres is meaningless. GPS.gov: GPS Accuracy

      Still, who am I to comment? I still can't work the system in my car properly!

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      Re: GPS, you've gone too far this time

      I know that if I look at the location history on my phone that there has always been strange spikes when I've been either at home or out & about. The spikes can literally be anything from tens of metres from my true location at the time, to tens of miles.

      I've always put this down to receiving a bad signal from one of the satellites. Perhaps this article explains things better?

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      Re: GPS, you've gone too far this time

      Do you have a problem remembering where you have been in that you need to check your location history?

      It's something I've never thougt of looking at, and even now I still won't bother.

      If they are walking around inside a 5M square I am surprised the GPS says that they have moved at all, except that they will have crossed the invisible lines in one direction but not necessarily in the other.... This is going nowhere... A bit like the GPS!
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      Re: GPS, you've gone too far this time

      It can be interesting to see how far somewhere was and to fact-check some things if you need to write up something.

      The location history might be more useful for your partner to see where you are or where you've been.

      Of course if it shows that you're miles from your true location, then it kind of makes the whole process a little unreliable.

      If you have an Android phone, just goggle 'location history' to see.

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      Re: GPS, you've gone too far this time

      I do not wish to spy on my partner's location nor, I hope, she on mine.

      I've got location turned off on both my phones, at least the GPS bit, saves power and Google don't know exactly where I am either.
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      Re: GPS, you've gone too far this time

      Perhaps a bad example, but it could always be useful for spying on the children.

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