Google Maps: Get lat/lng from known start location and given km - google-maps

Question

Let's say I know the start lat/lng of highway A and that a construction zone starts at km 175.
Is there a way to calculate the lat/lng at km 175? (Probably with direction service)

Answer 1

Mike Williams wrote an extension to the Google Maps Javascript API v2 (now deprecated and turned off) called epoly which contains the method .GetPointAtDistance, which does what you want.
example using a ported version of that library
partial source code of the GetPointAtDistance function (depends on extending the google.maps.LatLng object with distanceFrom method)
/* .GetPointAtDistance() returns a google.maps.LatLng at the specified distance *
* along the path. *
* The distance is specified in metres *
* Returns null if the path is shorter than that *
*/
// === A method which returns a google.maps.LatLng of a point a given distance along the path ===
// === Returns null if the path is shorter than the specified distance ===
google.maps.Polygon.prototype.GetPointAtDistance = function(metres) {
// some awkward special cases
if (metres == 0) return this.getPath().getAt(0);
if (metres < 0) return null;
if (this.getPath().getLength() < 2) return null;
var dist=0;
var olddist=0;
for (var i=1; (i < this.getPath().getLength() && dist < metres); i++) {
olddist = dist;
dist += this.getPath().getAt(i).distanceFrom(this.getPath().getAt(i-1));
}
if (dist < metres) {
return null;
}
var p1= this.getPath().getAt(i-2);
var p2= this.getPath().getAt(i-1);
var m = (metres-olddist)/(dist-olddist);
return new google.maps.LatLng( p1.lat() + (p2.lat()-p1.lat())*m, p1.lng() + (p2.lng()-p1.lng())*m);
}