android - How does the distanceTo() method of the Location class work internally?

Question

I've tried using distanceTo() between two Location objects, but AFAIK it isn't returning the correct values in all situations. Instead, I've written a custom method which returns what I think are the correct values.

float distanceBetween(Location l1, Location l2)
{
    float lat1= (float)l1.getLatitude();
    float lon1=(float)l1.getLongitude();
    float lat2=(float)l2.getLatitude();
    float lon2=(float)l2.getLongitude();
    float R = 6371; // km
    float dLat = (float)((lat2-lat1)*Math.PI/180);
    float dLon = (float)((lon2-lon1)*Math.PI/180);
    lat1 = (float)(lat1*Math.PI/180);
    lat2 = (float)(lat2*Math.PI/180);

    float a = (float)(Math.sin(dLat/2) * Math.sin(dLat/2) +
            Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2));
    float c = (float)(2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)));
    float d = R * c * 1000;

    return d;
}

Could anyone tell me what the difference is between my implementation and what distanceTo() is returning?

Answer 1

I'm pretty certain that Location.distanceTo() is correctly designed. It uses the WGS84 approach which is highly precise and accurate. I recently used Location.distanceTo() for a number of test cases and it was never wrong.

You haven't explained why you think the method doesn't work properly. Its possible you may be getting wrong values because you haven't set the correct latitude and longitude values for a Location object. Also, you can set up the source code of Android in your IDE and check the method for yourself. It is absolutely correct.

EDIT:

On that link you posted, there's an answer that concisely sums up the problem:

distanceTo() works correctly. The error is on your side, most probable algorithmic one, e.g if there is no GPS fix available, and the phone takes a GSM cell based location, this of course can be off by 1000m.

For your app that probably wants to sum up the distance travelled, take only GPS fixes, don't use another LocationProvider than GPS!

So the moral of the story is to make sure that you use only the GPS provider for distance calculations.

EDIT 2:

Google recommends using the Google Play Services Location API over the android.location API. The new API is supposed to be more accurate.

References:

1. Making Your App Location-Aware.