Anyone got a function to get distance between two lan/lon (using degrees)

That says it all. I could re-invent it (distance between lats is fairly constant, but distance between lon's varied a great deal based on lat!).

One the lan and lon distances are known, it's basically a right triangle, with the the distance I'm looking for being the third side, I think!

travis.vitek over 10 years ago

Haversine Formula

jim_m_58 over 10 years ago

Thanks Travis, but I was hoping for a code fragment! As I said, I could reinvent it, but if someone has a tested way to do it, I'd refer that. Even "back to Start" in the standard activities has it, so someone has done it, and my IDEAL would be a call in the position class that does it in a standard way, but it could be 10 FW versions from now until it's standardized!

I'm sure something like this would help if anyone wanted to build a geocaching app too, as the app can determine the current gps location, and give a distance to the destination, and we're not talking something that needs to be very complex, as the distances involved are a matter of miles, and not 100's of miles!

travis.vitek over 10 years ago

You aren't reinventing anything. The equations are right there. If you must copy code... http://rosettacode.org/wiki/Haversine_formula

travis.vitek over 10 years ago

using Toybox.Math as Math;

    // en.wikipedia.org/.../Haversine_formula
    // returns hav(theta) = sin^2(theta / 2) = (1 - cos(theta)) / 2
    function haversin(theta) {
        return (1 - Math.cos(theta)) / 2;
    }

    // en.wikipedia.org/.../Great-circle_distance
    // p1 and p2 are start and end positions in radians
    // returns distance between points p1 and p2 in kilometers.
    function distance(p1, p2) {
        var h = haversin(p2[0] - p1[0]) + Math.cos(p1[0]) * Math.cos(p2[0]) * haversin(p2[1] - p1[1]);
        return 2 * 6371 * Math.asin(Math.sqrt(h));
    }

Usage example...

if (info.startLocation != null && info.currentLocation != null) {
    var p1 = info.startLocation.toRadians();
    var p2 = info.currentLocation.toRadians();
    Sys.println(1000* distance(p1, p2));
}

jim_m_58 over 10 years ago

Thanks travis. Looks like the distance returned in in Km's, correct?

update: I just ran a test on a real device, and I can't see what's being returned (the 6371 made me think metric). It's not Km, meters, or even degrees. It seems to map to how many houses I have to walk past! :)

Former Member over 10 years ago

I do it this way:

// computes the direct distance between two points

// distance = sqrt(dx * dx + dy * dy)

// with distance:  in km 

// dx = 111.3 * cos(lat) * (lon1 - lon2)

// lat = (lat1 + lat2) / 2 * 0.01745

// dy = 111.3 * (lat1 - lat2)

// lat1, lat2, lon1, lon2: Breite, Länge in Grad

// 1° = &#960;/180 rad &#8776; 0.01745

function computeDistance (pos1, pos2) {

  var lat1, lat2, lon1, lon2, lat, lon;

  var dx, dy, distance;

  

  lat1 = pos1.toDegrees()[0].toFloat();

  lon1 = pos1.toDegrees()[1].toFloat();

  lat2 = pos2.toDegrees()[0].toFloat();

  lon2 = pos2.toDegrees()[1].toFloat();

    

  lat = (lat1 + lat2) / 2 * 0.01745;

  dx = 111.3 * Math.cos(lat) * (lon1 - lon2); 

  dy = 111.3 * (lat1 - lat2);

  distance = 1000 * Math.sqrt(dx * dx + dy * dy);

  distance = distance.toNumber();

  strokelength = Lang.format("$1$ m", [distance]);

  validstrokeposition = true;

  Sys.println(strokelength);

}

jim_m_58 over 10 years ago

Thanks planetscooter - I'll give it a try!

update: This seems to work great! A few minor changes in how things are passed and returned, but that's it!

travis.vitek over 10 years ago

It's kilometers, and the input arrays are lat/lon in radians. I'm away from home, so I don't have access to my code that uses it (and calculates the initial bearing)

xuxumatu over 6 years ago in reply to travis.vitek

why info.currentHeading gives negative numbers??

jim_m_58 over 6 years ago in reply to xuxumatu

It's in radians with north being 0, so it's +/- based on north