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Can I use your GPS SDK to calculate the distance between two coordinates ?

Yes, it is possible to do high precision distance calculations using the GPS Toolkit. Instead of the Great Circle formula, it uses Vincenty's formula which far more accurate.

The function to calculate the distance takes the source and destination coordinates in latitude / longitude format. If your position is in Northing / Easting, you have to convert it to WGS84 first. Below you can find the sample code to calculate distance:

' This demo shows how to calculate the distance between two WGS84 coordinates
' and how to use the ConvertUnits functions to convert this distance to various distance units like miles and feet.
' For more information on how to use the Eye4Software GPS Toolkit with VBScript, 
' visit http://www.eye4software.com/products/gpstoolkit/source#vbscript

Option Explicit

Dim objUtilities, objConstants

Dim srcLatitude
Dim srcLongitude
Dim dstLatitude 
Dim dstLongitude
Dim dblDistance

srcLatitude   = 51.90
srcLongitude  =  4.24
dstLatitude   = 51.89
dstLongitude  =  4.93
dblDistance   =  0.00

' Create intance of GpsUtilities object
Set objUtilities	= CreateObject ( "Eye4Software.GpsUtilities" )

' Create instance of GpsConstants object (distance units)
Set objConstants  = CreateObject ( "Eye4Software.GpsConstants" )

WScript.Echo "Eye4Software GPS Toolkit " & objUtilities.Version & " - Distance and Units Demo" 
WScript.Echo


' Calculate Distance, Azimuth and Reverse Azimuth
dblDistance       = objUtilities.CalculateDistance ( srcLatitude, srcLongitude, dstLatitude, dstLongitude )


WScript.Echo "Result           = " & objUtilities.LastErrorDescription
WScript.Echo
WScript.Echo "Distance (m)     = " & FormatNumber ( dblDistance, 2, 0, 0, 0 )
WScript.Echo "Distance (km)    = " & FormatNumber ( objUtilities.ConvertUnits ( objConstants.GPS_PROJECTION_UNITS_M, objConstants.GPS_PROJECTION_UNITS_KM, dblDistance ), 2, 0, 0, 0 )
WScript.Echo "Distance (miles) = " & FormatNumber ( objUtilities.ConvertUnits ( objConstants.GPS_PROJECTION_UNITS_M, objConstants.GPS_PROJECTION_UNITS_MI, dblDistance ), 2, 0, 0, 0 )
WScript.Echo "Distance (NM)    = " & FormatNumber ( objUtilities.ConvertUnits ( objConstants.GPS_PROJECTION_UNITS_M, objConstants.GPS_PROJECTION_UNITS_NM, dblDistance ), 2, 0, 0, 0 )
WScript.Echo "Distance (feet)  = " & FormatNumber ( objUtilities.ConvertUnits ( objConstants.GPS_PROJECTION_UNITS_M, objConstants.GPS_PROJECTION_UNITS_FT, dblDistance ), 0, 0, 0, 0 )
WScript.Echo 
WScript.Echo "Azimuth          = " & FormatNumber ( objUtilities.CalculateAzimuth ( srcLatitude, srcLongitude, dstLatitude, dstLongitude ), 1, 0, 0, 0 )
WScript.Echo "Azimuth (rev)    = " & FormatNumber ( objUtilities.CalculateAzimuth ( dstLatitude, dstLongitude, srcLatitude, srcLongitude ), 1, 0, 0, 0 )
WScript.Echo
WScript.Echo "Ready."