## 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.

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.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>
{
return (deg * Math.PI / 180.0);
}

}
}``` 