Calculate distance between two positions

Former Member over 10 years ago

Hello together,

I'm currently trying to calculate the distance between two positions in a widget. I've found this javaScript algorithm to do so but am not really sure how to translate it to MC:

var R = 6371000; // metres
var φ1 = lat1.toRadians();
var φ2 = lat2.toRadians();
var Δφ = (lat2-lat1).toRadians();
var Δλ = (lon2-lon1).toRadians();

var a = Math.sin(Δφ/2) * Math.sin(Δφ/2) +
Math.cos(φ1) * Math.cos(φ2) *
Math.sin(Δλ/2) * Math.sin(Δλ/2);
var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

var d = R * c;

The problem is these two lines:

var Δφ = (lat2-lat1).toRadians();
var Δλ = (lon2-lon1).toRadians();

since I dont know how to convert the two subtracted degrees to radians.

Any help would be appreciated.

Former Member over 10 years ago

This is what I do in my App:

// 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]);
}

And call it like

if( Toybox has :Position ) {
  	loc_info = Position.getInfo();
  	if (loc_info != null) {
  		if (validstrokeposition == true) {
  		  thisstrokeat = loc_info.position;
  		  computeDistance(laststrokeat, thisstrokeat);
  		}  
  	}
  	validstrokeposition = true;
 	laststrokeat = loc_info.position;
 	
  }

travis.vitek over 10 years ago

The problem is these two lines:

var Δφ = (lat2-lat1).toRadians();
var Δλ = (lon2-lon1).toRadians();

since I dont know how to convert the two subtracted degrees to radians.

If you have Position.Location object, it has a toRadians(). Most likely you'll have a Location object to start, so this isn't an issue. If you have the latitude and longitude in degrees, you just need to convert from degrees to radians.

function degrees_to_radians(deg) {
    return deg * Math.PI / 180;
}

The conversion looks like...

function Geodetic_distance_deg(lat1, lon1, lat2, lon2) {
  lat1 = degrees_to_radians(lat1);
  lon1 = degrees_to_radians(lon1);
  lat2 = degrees_to_radians(lat2);
  lon2 = degrees_to_radians(lon2);

  return Geodetic_distance_rad(lat1, lon1, lat2, lon2)
}

function Geodetic_distance_rad(lat1, lon1, lat2, lon2) {
  var dy = (lat2-lat1);
  var dx = (lon2-lon1);

  var sy = Math.sin(dy / 2);
  sy *= sy;

  var sx = Math.sin(dx / 2);
  sx *= sx; 

  var a = sy + Math.cos(lat1) * Math.cos(lat2) * sx;

  // you'll have to implement atan2
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

  var R = 6371000; // radius of earth in meters
  return R * c;
}

Former Member over 10 years ago

Thanks for the answers. Helped me a lot.