Calculate distance between two coordinates on earth

This is part of the latest feature of our Advanced Sitecore Google Maps (WCAG Edition). I have added a new feature to find the nearest markers based on an address.

Find Nearest Markers

The caclculation is based on the approximate birds-flight distance between 2 latitude/longitude coordinates. Google Maps has a JavaScript function to give you the numbers, but since our Google Maps implementation will work without JavaScript I have to do the math in C#.

The calculation needs to return the distance in either kilometers or miles, so I have created an enumeration for this:

 public enum MeasureUnits { Miles, Kilometers }; 

 The rest is pure math and is based on an approximated earth radius of 3960 miles (or 6371 kilometers). The class is basically the same Coordinate class I used in the Google Maps Polyline Encoding article. The class is extended with the DistanceFrom function that calculates the distance to another Coordinate.

namespace PT.GoogleMaps.Core
{
  public class Coordinate
  {
    /// <summary>
    /// The units of measure when calculating distance between 2 coordinates
    /// </summary>
    public enum MeasureUnits { Miles, Kilometers };
    /// <summary>
    /// Gets or sets the latitude of the coordinate.
    /// </summary>
    public double Latitude { get; set; }

    /// <summary>
    /// Gets or sets the longitude of the coordinate.
    /// </summary>
    public double Longitude { get; set; }

    /// <summary>
    /// Initializes a new instance of the <see cref="Coordinate"/> class.
    /// </summary>
    /// <param name="latitude">The latitude.</param>
    /// <param name="longitude">The longitude.</param>
    public Coordinate(double latitude, double longitude)
    {
      Latitude = latitude;
      Longitude = longitude;
    }

    /// <summary>
    /// Calculates the approximate birds-flight distance between this coordinate and the coordinate in the parameter
    /// </summary>
    /// <param name="coordinate">The coordinate.</param>
    /// <param name="units">The units to measure in</param>
    /// <returns></returns>
    public double DistanceFrom(Coordinate coordinate, MeasureUnits units)
    {
      double earthRadius = (units == MeasureUnits.Miles) ? 3960 : 6371;
      double dLat = Deg2Rad(coordinate.Latitude - Latitude);
      double dLon = Deg2Rad(coordinate.Longitude - Longitude);
      double a = Math.Sin(dLat / 2) *
                 Math.Sin(dLat / 2) +
                 Math.Cos(Deg2Rad(Latitude)) *
                 Math.Cos(Deg2Rad(coordinate.Latitude)) *
                 Math.Sin(dLon / 2) *
                 Math.Sin(dLon / 2);
      double c = 2 * Math.Asin(Math.Min(1, Math.Sqrt(a)));
      double d = earthRadius * c;
      return d;
    }

    /// <summary>
    /// Converts from Degrees to Radians
    /// </summary>
    /// <param name="deg">The degrees</param>
    /// <returns>The radians</returns>
    private double Deg2Rad(double deg)
    {
      return (deg * Math.PI / 180.0);
    }

  }
}