GPS to calculate distance between two points on windows phone 7

Question

i am using GPS to calculate distance between two points i.e. i am using windows phone as a tape measure but when i start i dont get the correct value infact even if i am standing still it gives me hundreds of meter

here is my code

      myWatcher.StatusChanged += new EventHandler<GeoPositionStatusChangedEventArgs>(myWatcher_StatusChanged);
        myWatcher.PositionChanged += new EventHandler<GeoPositionChangedEventArgs<GeoCoordinate>>(myWatcher_PositionChanged);
        myWatcher.MovementThreshold = 1;   

void myWatcher_PositionChanged(object sender, GeoPositionChangedEventArgs<GeoCoordinate> e)
    {
        double tempf = e.Position.Location.Latitude;
        double temps = e.Position.Location.Longitude;
        if (count2 == 0)
        {
            FirstLocation = new GeoCoordinate(tempf, temps);
            count2++;
        }
        else
        {
             double distanceInMeter;
            GeoCoordinate currentLocation;
                currentLocation = new GeoCoordinate(tempf, temps);
                distanceInMeter = currentLocation.GetDistanceTo(FirstLocation);                   

                if (App.flag == 0)
                {
                    textBlock1.Text = distanceInMeter.ToString() + " m";
                    double distanceInCm = distanceInMeter * 100;
                    textBlock2.Text = distanceInCm .ToString() + " cm";
                }
                else if (App.flag == 1)
                {
                    double distanceInInch = distanceInMeter * 39.3701;
                    textBlock1.Text = distanceInInch.ToString() + " in";
                    double distanceInFoot = distanceInMeter * 3.28084;
                    textBlock2.Text = distanceInFoot.ToString() + " ft";
                }
        }

    }

as a tape measure? gps is only accurate to around 8 meters in the *best case*. if you are getting tower assisted gps instead of true gps, 100's of meters is probably best case. — John Gardner, Dec 11 '12 at 18:13
so far accuracy is not an issue. i run this app and the reading at current location changes continuously while i have set movementthreshold to 1 means 1 meter of distance needs to be covered to call PositionChanged event but reading keeps on changing even though i am still — Hamza, Dec 11 '12 at 19:24
how can you say "accuracy is not an issue" if your entire question is about accuracy? if any individual measurement of GPS is only good to within (at best!) 3 meters, any 2 measurements from the *exact same place* could be (at best!) 6 meters apart! again, **at best**. gps is best for measuring distances on the *planet*, not on a *piece of paper*. — John Gardner, Dec 11 '12 at 19:32
no thats not what i meant its just that reight now i have not reached to accurate measurement phase and i know 3 - 4 meters is accuracy and i'll make the app for that but the current issue is that even if i am standing still i get hundreds of meter of distance although it is inacurate but it should give almost 0 at the current location. see my code above — Hamza, Dec 12 '12 at 03:56
did you actually look at the 2 coords you get from your 2 "standing still" calls? did you plug them into a map to see what you got? did you calculate the distance between those calls some other way not using this call? Like I've been saying, any 2 calls could give you 2 answers. the first call might get you tower assisted coords, which are very innacurate. the second call might get a proper gps fix, giving you a more precise answer, which is different than the first! — John Gardner, Dec 12 '12 at 17:49

score 1 · Answer 1 · answered Dec 11 '12 at 18:19

1

At best, a GPS is only accurate to 3 meters. The more likely error is 5 - 10 meters.

You're going to have to augment your GPS readings to get better accuracy.

answered Dec 11 '12 at 18:19

Gilbert Le Blanc

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The GPS Standard Positioning Service (which is all you got unless you've got special military hardware), at best offers +/- 7.8 meters in 3 dimensions at a 95% confidence level. That's a sphere of uncertainty 52 feet in diameter. You can read the current edition of the standard at http://www.gps.gov/technical/ps/. That's the best case. There are many other sources of entropy that reduce the precision of the solution, including hardware/software issues on the part of the GPS receiver, atmospheric and other external factors, etc. – Nicholas Carey Dec 11 '12 at 18:25

score 1 · Answer 2 · answered Dec 11 '12 at 19:24

so far accuracy is not an issue. i run this app and the reading at current location changes continuously while i have set movementthreshold to 1 means 1 meter of distance needs to be covered to call PositionChanged event but reading keeps on changing even though i am still

score 0 · Answer 3 · answered Dec 11 '12 at 18:32

The other posters here are generally correct: GPS will not give you the accuracy you need. At least not alone!

Assuming you did have a good satellite fix, you could get an accuracy of centimeters, but this would require post-processing with information from special fixed stations or a live feed of data from such a station. The buzzwords here are "real-time phase differential", "real-time kinematic", "phase-differential post-processing".

This site has some details.

So, if you can get access to feed, it may be possible. But it will not be as easy as you probably hoped.

GPS to calculate distance between two points on windows phone 7

3 Answers3

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