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Sample Codes (Visual Basic .NET)
import System
':::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
':::                                                                         :::
':::  This routine calculates the distance between two points (given the     :::
':::  latitude/longitude of those points). It is being used to calculate     :::
':::  the distance between two locations using GeoDataSource (TM) prodducts  :::
':::                                                                         :::
':::  Definitions:                                                           :::
':::    South latitudes are negative, east longitudes are positive           :::
':::                                                                         :::
':::  Passed to function:                                                    :::
':::    lat1, lon1 = Latitude and Longitude of point 1 (in decimal degrees)  :::
':::    lat2, lon2 = Latitude and Longitude of point 2 (in decimal degrees)  :::
':::    unit = the unit you desire for results                               :::
':::           where: 'M' is statute miles (default)                         :::
':::                  'K' is kilometers                                      :::
':::                  'N' is nautical miles                                  :::
':::                                                                         :::
':::  Worldwide cities and other features databases with latitude longitude  :::
':::  are available at http://www.geodatasource.com                          :::
':::                                                                         :::
':::  For enquiries, please contact sales@geodatasource.com                  :::
':::                                                                         :::
':::  Official Web site: http://www.geodatasource.com                        :::
':::                                                                         :::
':::              GeoDataSource.com (C) All Rights Reserved 2017             :::
':::                                                                         :::
':::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Public Function distance(ByVal lat1 As Double, ByVal lon1 As Double, ByVal lat2 As Double, ByVal lon2 As Double, ByVal unit As Char) As Double
    Dim theta As Double = lon1 - lon2
    Dim dist As Double = Math.Sin(deg2rad(lat1)) * Math.Sin(deg2rad(lat2)) + Math.Cos(deg2rad(lat1)) * Math.Cos(deg2rad(lat2)) * Math.Cos(deg2rad(theta))
    dist = Math.Acos(dist)
    dist = rad2deg(dist)
    dist = dist * 60 * 1.1515
    If unit = "K" Then
        dist = dist * 1.609344
    ElseIf unit = "N" Then
        dist = dist * 0.8684
    End If
    Return dist
End Function

Private Function deg2rad(ByVal deg As Double) As Double
    Return (deg * Math.PI / 180.0)
End Function

Private Function rad2deg(ByVal rad As Double) As Double
    Return rad / Math.PI * 180.0
End Function

The sample code is licensed under LGPLv3.