Fixed distance computation between two coordinates

index.js

@@ -54,36 +54,95 @@ module.exports = function(point1, point2, units) {
   var dLon = toRad(coordinates2[0] - coordinates1[0]);
   var lat1 = toRad(coordinates1[1]);
   var lat2 = toRad(coordinates2[1]);
-  var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
-          Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2);
-  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
+  var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
+    Math.sin(dLon / 2) * Math.sin(dLon / 2) *
+    Math.cos(lat1) * Math.cos(lat2);
+  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
 
   var R;
-  switch(units) {
+  switch (units) {
     case 'miles':
-      R = 3960;
-      break;
     case 'kilometers':
     case 'kilometres':
-      R = 6373;
+    case 'meters':
+    case undefined:
+      R = getEarthRadius(lat1, units);
       break;
     case 'degrees':
       R = 57.2957795;
       break;
     case 'radians':
       R = 1;
       break;
-    case undefined:
-      R = 6373;
-      break;
     default:
       throw new Error('unknown option given to "units"');
   }
 
+  // Round result.
   var distance = R * c;
-  return distance;
+
+  switch (units) {
+    case 'miles':
+    case 'kilometers':
+    case 'kilometres':
+    case undefined:
+      distance = Math.floor(distance * 1000) / 1000;
+      break;
+
+    case 'meters':
+      distance = Math.floor(distance);
+      break;
+
+    default:
+      break;
+  }
+
+  return Number(distance);
 };
 
 function toRad(degree) {
   return degree * Math.PI / 180;
 }
+
+function kmToMiles(kilometers) {
+  return kilometers * 0.621371192;
+}
+
+function metersToMiles(meters) {
+  return meters * 0.000621371192;
+}
+
+function getEarthRadius(latdeg, units) {
+
+  var radiusMajorInMeters = 6378137.0;
+  var radiusMinorInMeters = 6356752.3142;
+  var radiusMajor = 0;
+  var radiusMinor = 0;
+
+  switch (units) {
+    case 'miles':
+      radiusMajor = metersToMiles(radiusMajorInMeters);
+      radiusMinor = metersToMiles(radiusMinorInMeters);
+      break;
+    case 'kilometers':
+    case 'kilometres':
+    case undefined:
+      radiusMajor = radiusMajorInMeters / 1000;
+      radiusMinor = radiusMinorInMeters / 1000;
+      break;
+    case 'meters':
+      radiusMajor = radiusMajorInMeters;
+      radiusMinor = radiusMinorInMeters;
+      break;
+    default:
+      throw new Error('unknown option given to "units"');
+  }
+
+  // http://en.wikipedia.org/wiki/Earth_radius
+  var An = radiusMajor * radiusMajor * Math.cos(latdeg);
+  var Bn = radiusMinor * radiusMinor * Math.sin(latdeg);
+  var Ad = radiusMajor * Math.cos(latdeg);
+  var Bd = radiusMinor * Math.sin(latdeg);
+
+  return Math.sqrt((An * An + Bn * Bn) / (Ad * Ad + Bd * Bd));
+}

test.js

@@ -1,25 +1,77 @@
 var test = require('tape');
 var distance = require('./');
 
-test('distance', function(t){
+test('distance', function(t) {
   var pt1 = {
     "type": "Feature",
-    "geometry": {"type": "Point", "coordinates": [-75.343, 39.984]}
+    "geometry": {
+      "type": "Point",
+      "coordinates": [-75.343, 39.984]
+    }
   };
   var pt2 = {
     "type": "Feature",
-    "geometry": {"type": "Point", "coordinates": [-75.534, 39.123]}
+    "geometry": {
+      "type": "Point",
+      "coordinates": [-75.534, 39.123]
+    }
   };
 
-  t.equal(distance(pt1, pt2, 'miles'), 60.37218405837491, 'miles');
-  t.equal(distance(pt1, pt2, 'kilometers'), 97.15957803131901, 'kilometers');
-  t.equal(distance(pt1, pt2, 'kilometres'), 97.15957803131901, 'kilometres');
+  // Center point
+  var pt3 = {
+    "type": "Feature",
+    "geometry": {
+      "type": "Point",
+      "coordinates": [-73.5719678, 45.5260794]
+    }
+  };
+
+  // On the circle radius (center + 400 meters + 0 degres)
+  var pt4 = {
+    "type": "Feature",
+    "geometry": {
+      "type": "Point",
+      "coordinates": [-73.57196780000001, 45.52968474026626]
+
+    }
+  };
+
+  // On the circle radius too (center + 400 meters + 270 degres)
+  var pt5 = {
+    "type": "Feature",
+    "geometry": {
+      "type": "Point",
+      "coordinates": [-73.57711398872225, 45.52607928446446]
+    }
+  };
+
+  /* Test # 01 */
+  t.equal(distance(pt1, pt2, 'miles'), 60.337, 'miles');
+  /* Test # 02 */
+  t.equal(distance(pt1, pt2, 'kilometers'), 97.103, 'kilometers');
+  /* Test # 03 */
+  t.equal(distance(pt1, pt2, 'kilometres'), 97.103, 'kilometres');
+  /* Test # 04 */
   t.equal(distance(pt1, pt2, 'radians'), 0.015245501024842149, 'radians');
+  /* Test # 05 */
   t.equal(distance(pt1, pt2, 'degrees'), 0.8735028650863799, 'degrees');
-  t.equal(distance(pt1, pt2), 97.15957803131901, 'default=kilometers');
+  /* Test # 06 */
+  t.equal(distance(pt1, pt2), 97.103, 'default=kilometers');
+  /* Test # 07 */
+  t.equal(distance(pt3, pt4, 'meters'), 400, 'meters');
+  /* Test # 08 */
+  t.equal(distance(pt3, pt4, 'kilometers'), 0.400, 'kilometers');
+  /* Test # 09 */
+  t.equal(distance(pt3, pt5, 'meters'), 400, 'meters');
+  /* Test # 10 */
+  t.equal(distance(pt3, pt5, 'kilometers'), 0.400, 'kilometers');
+  /* Test # 11 */
+  t.equal(distance(pt3, pt4, 'meters'), distance(pt3, pt5, 'meters'));
+  /* Test # 12 */
+  t.equal(distance(pt3, pt4, 'kilometers'), distance(pt3, pt5, 'kilometers'));
 
   t.throws(function() {
-      distance(pt1, pt2, 'blah');
+    distance(pt1, pt2, 'blah');
   }, 'unknown option given to units');
 
   t.end();