Google maps - get latlng of X% along a straight polyline

Question

I need to get the latitude/longitude of a position at X percentage along a straight polyline segment between two other lat/lng points.

The closest I have come so far is using the following (for 40% along the line):

google.maps.geometry.spherical.interpolate(startLatLng, endLatLng, 0.4);

This works perfectly for short distances, however for a long polyline segment the latlng returned is outside of where the polyline travels, since it's working out the shortest distance along the earth's surface instead of the shortest distance across a flat map.

Any help on this would be appreciated, hopefully I'm just missing something obvious.

Answer 1

There must be some proven and tested method out there to do this, but here is a way you could do it with the Maps API:

1. Convert the lat/lng coordinates to pixels
2. Interpolate in the pixel plane
3. Convert back to lat/lng

This is mostly pretty straightforward; the one hassle is figuring out how to deal with lines that cross the 180 meridian or a pole.

Here's some semi-tested code that may give you a place to start:

function mercatorInterpolate( map, latLngFrom, latLngTo, fraction ) {
    // Get projected points
    var projection = map.getProjection();
    var pointFrom = projection.fromLatLngToPoint( latLngFrom );
    var pointTo = projection.fromLatLngToPoint( latLngTo );
    // Adjust for lines that cross the 180 meridian
    if( Math.abs( pointTo.x - pointFrom.x ) > 128 ) {
        if( pointTo.x > pointFrom.x )
            pointTo.x -= 256;
        else
            pointTo.x += 256;
    }
    // Calculate point between
    var x = pointFrom.x + ( pointTo.x - pointFrom.x ) * fraction;
    var y = pointFrom.y + ( pointTo.y - pointFrom.y ) * fraction;
    var pointBetween = new google.maps.Point( x, y );
    // Project back to lat/lng
    var latLngBetween = projection.fromPointToLatLng( pointBetween );
    return latLngBetween;
}

I'm not 100% sure about the part that handles lines that cross the 180 meridian, but it works OK in the few quick tests I tried.

Answer 2

See this discussion in the Google Maps API v3 group

And this example from it

This suggested code takes the projection into account for the mid-point:

google.maps.event.addListener(map, 'projection_changed', function() {
  var projection = map.getProjection();
  if (!projection) return;

  // Project
  var startLatLng = startMarker.getPosition();
  var endLatLng = endMarker.getPosition();
  var startPoint = projection.fromLatLngToPoint(startLatLng);
  var endPoint = projection.fromLatLngToPoint(endLatLng);

  // Average
  var midPoint = new google.maps.Point(
      (startPoint.x + endPoint.x) / 2,
      (startPoint.y + endPoint.y) / 2);

  // Unproject
  var midLatLng = projection.fromPointToLatLng(midPoint);
  midMarker.setPosition(midLatLng);

});