Google distance between two GPS points using GMap3.

There is a method in GMap3. It's a static method of the google.maps.geometry.spherical namespace.

INPUT: two Latitude+Longitude [LatLong] objects.
The function will utilize a default Earth radius of 6378137 meters. If necessary,
the default radius can be overridden with a custom value .

In the HTML <Head>, be sure to include:

<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=falsev=3&libraries=geometry" />


The call will look like this:

google.maps.geometry.spherical.computeDistanceBetween (latLongA, latLongB);

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NOTE: for more info on the Google GMap3, see http://gmap3.net/quickstart.html
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Example using GPS latitude/longitude of 2 points.  (Distance results are expressed in meters.)

var latitude1 = 39.06;
var longitude1 = -0.26;
var latitude2 = 40.41;
var longitude2 = -3.67;

var distance = google.maps.geometry.spherical.computeDistanceBetween(new google.maps.LatLng(latitude1, longitude1), new google.maps.LatLng(latitude2, longitude2));

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Alternatively... Add this to the beginning of your JavaScript code:
google.maps.LatLng.prototype.distanceFrom = function(latlng) {
  var lat = [this.lat(), latlng.lat()]
  var long = [this.lng(), latlng.lng()]
  var Rad = 6378137;
  var dLat = (lat[1]-lat[0]) * Math.PI / 180;
  var dLong = (long[1]-long[0]) * Math.PI / 180;
  var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
  Math.cos(lat[0] * Math.PI / 180 ) * Math.cos(lat[1] * Math.PI / 180 ) *
  Math.sin(dLong/2) * Math.sin(dLong/2);
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
  var d = Rad * c;
  return Math.round(d);
}

...then use the 'distanceFrom()' function like this:

var loc1 = new GLatLng(52.5773139, 1.3712427);
var loc2 = new GLatLng(52.4788314, 1.7577444);
var dist = loc2.distanceFrom(loc1);
alert(dist/1000);

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Here is the c# implementation of the formula:

 public class DistanceAlgorithm
{
    const double PIx = 3.141592653589793;
    const double RADIO = 6378.16;

    /// <summary>
    /// This class is static -it cannot be instantiated.
    /// </summary>
    private DistanceAlgorithm() { }

    /// <summary>
    /// Convert degrees to Radians
    /// </summary>
    /// <param name="x">Degrees</param>
    /// <returns>The equivalent in radians</returns>
    public static double Radians(double x)
    {
        return x * PIx / 180;
    }

    /// <summary>
    /// Calculate the distance between two places.
    /// </summary>
    /// <param name="lon1"></param>
    /// <param name="lat1"></param>
    /// <param name="lon2"></param>
    /// <param name="lat2"></param>
    /// <returns></returns>
    public static double DistanceBetweenPlaces(
        double lon1,
        double lat1,
        double lon2,
        double lat2)
    {
        double dlon =  Radians(lon2 - lon1);
        double dlat =  Radians(lat2 - lat1);

        double a = (Math.Sin(dlat / 2) * Math.Sin(dlat / 2)) + Math.Cos(Radians(lat1)) * Math.Cos(Radians(lat2)) * (Math.Sin(dlon / 2) * Math.Sin(dlon / 2));
        double angle = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
        return (angle * RADIO) * 0.62137;//distance in miles
    }

}   

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// NOTE: p1 and p2 are google.maps.LatLng(x,y) objects

function calcDistance(p1, p2) {
          var d = (google.maps.geometry.spherical.computeDistanceBetween(p1, p2) / 1000).toFixed(2);
          console.writeline(d); 
         var name = console.ReadLine();  
}


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// Using PHP, you can calculate the distance using this simple 'calculate_distance' function
// to calculate distance between two lat & long

function calculate_distance($lat1, $lon1, $lat2, $lon2, $unit='N')
{
  $theta = $lon1 - $lon2;
  $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) +  cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
  $dist = acos($dist);
  $dist = rad2deg($dist);
  $miles = $dist * 60 * 1.1515;
  $unit = strtoupper($unit);

  if ($unit == "K") {
    return ($miles * 1.609344);
  } else if ($unit == "N") {
      return ($miles * 0.8684);
    } else {
        return $miles;
      }
}

// function ends here
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