Retrieving destination coordinates from direction services by distance - google-maps

Question

I need to retrieve a destination's coordinates using the google maps api directions service. I already have the starting point coordinates, however instead of specifying an ending point in coordinates, I wish to retrieve the coordinates by specifying a distance (in km).
So I guess my question is the following: is it possible to retrieve the destination latlong coordinates (based/calculated on the road's distance and not directional/straight line) by specifying a distance (amount in km) with the directions service or perhaps any alternative way?
I have an image illustration, however unfortunately am unable to attach to this question as I do not have enough reputation. If my question is unclear in any way, or you wish to see the illustration then please contact me and I'll send it off.

Answer 1

I don't think you can do this as the request parameters say that origin and destination parameters are required.

Answer 2

I beliave it will help someone.
There is a method to get coordinates in the google maps library:
google.maps.geometry.spherical.computeOffset(fromCoordinates, distanceInMeters, headingInDegrees)

Answer 3

I believe you are correct. There doesn't seem to be any current method in the api which would allow you to do the following.
Instead I looped through the coordinates returned from the directions service call, and used a function to calculate the distance between coordinates. However even this was not accurate enough as the coordinates returned also seemed to be aggregated and doesn't return an accurate value/distance when calculating the distances between each coordinate as they are aggregated and therefore each coordinate is not necessary along the road.
To work around the above issue, I ended up adding a click event, and plotted the coordinates along the road myself and then stored them in a local json file which I cache and call using an xmlhttprequest.
Fortunately, for my situation I only need to calculate the distance between point A & B on one individual road, so my alternative won't work in cases when you're using multiple or generic roads/locations. You could instead use the first method described, given that you're happy to live with the aggregated data and an in-accurate calculation.
Below are the functions used to calculate the distances between coordinates and then also the final calculation to find the point & coordinates between the final two points. Please note this code relies on and uses jQuery methods.
1. Calculate distance (in meters) between two coordinates
function pointDistance( begin, end )
{
var begin = { lat: begin[0], long: begin[1] },
end = { lat: end[0], long: end[1] };
// General calculations
var earthRadius = 6371, //km
distanceLat = (end.lat - begin.lat).toRad(),
distanceLong = (end.long - begin.long).toRad();
// Convert lats to radiants
begin.lat = begin.lat.toRad();
end.lat = end.lat.toRad();
// Calculation
var a = Math.sin(distanceLat / 2) * Math.sin(distanceLat / 2) +
Math.sin(distanceLong / 2) * Math.sin(distanceLong / 2) * Math.cos(begin.lat) * Math.cos(end.lat);
var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
var distance = (earthRadius * c) - 0.000536;
return (distance * 1000);
}
2. Fetch coordinate of final A-B coordinate (based on percentage remaining). The 'matrix' variable is a json array of coordinates.
function getCoordinates( totalDistance )
{
var lastPoint = { lat: null, long: null },
total = parseFloat(0),
position = { start: null, end: null, distance: 0 };
$(matrix).each(function()
{
if ( lastPoint.lat == null )
{
lastPoint = { lat: this[0], long: this[1] };
return;
}
var distance = pointDistance([lastPoint.lat, lastPoint.long], [this[0], this[1]]);
total = total + distance;
if ( (total / 1000) >= totalDistance )
{
position.start = new google.maps.LatLng(lastPoint.lat, lastPoint.long);
position.end = new google.maps.LatLng(this[0], this[1]);
position.distance = total;
return false;
}
lastPoint = { lat: this[0], long: this[1] };
});
return position;
}
3. Convert numeric degrees to radians
if ( typeof(Number.prototype.toRad) === 'undefined' ) {
Number.prototype.toRad = function() {
return this * Math.PI / 180;
}
}
Hope the following helps any one with the same or simular problem. I haven't investigated this as I've had no need to, but, perhaps if you're dealing with google's paid services, they don't aggregate the data returned by the call?