Bug 515335. Distance calculation between 2 geocoordinates incorrect for points not on the equator. r=dougt.

--HG--
extra : rebase_source : 7f73773d16fb2ecee1f57e0c106b014c2f424de9

dom/src/geolocation/nsGeolocation.cpp

@@ -540,11 +540,17 @@ nsGeolocationService::IsBetterPosition(nsIDOMGeoPosition *aSomewhere)
   NS_ENSURE_SUCCESS(rv, PR_FALSE);
 
   // check to see if there has been a large movement
-  double delta = fabs(newLat - oldLat) + fabs(newLon + oldLon);
+  // Use spherical law of cosines to calculate difference
+  // Not quite as correct as the Haversine but simpler and cheaper
+  double radsInDeg = 3.14159265 / 180.0;
 
-  // Convert to meters. 1 second of arc of latitude (or longitude at the
-  // equator) is 1 nautical mile or 1852m.
-  delta *= 60 * 1852;
+  double rNewLat = newLat * radsInDeg;
+  double rNewLon = newLon * radsInDeg;
+  double rOldLat = oldLat * radsInDeg;
+  double rOldLon = oldLon * radsInDeg;
+
+  // WGS84 equatorial radius of earth = 6378137m
+  double delta = acos( (sin(rNewLat) * sin(rOldLat)) + (cos(rNewLat) * cos(rOldLat) * cos(rOldLon - rNewLon)) ) * 6378137; 
 
   // The threshold is when the distance between the two positions exceeds the
   // worse (larger value) of the two accuracies.