How to Build in a Simple Distance Checker

This method will only calculate the distance in Kilometres (divide the result by 1.609344 for miles) “as the crow files” but it is extremely useful when, for instance, you are marketing everyone within a fixed radius.

So how do we do it? The method uses the “Haversine” formula which assumes that the Earth is a sphere. OK we know that the Earth is not a perfect sphere, but when you are checking between two map co-ordinates it is more than accurate enough for these purposes.

OK so without going into the technicalities of how the formula works, we shall get straight on with the coding requirements for VBA.

You will need a table in your application containing all the UK postcode variations and the X & Y axis co-ordinates. We would be very happy to provide a copy.

The following VBA Code attached to the “Calculate driving distance Distance” click event is used in a simple form where the user enters the start and end postcodes and the distance is calculated.

txtPostCodeStart and txtPostCodeEnd are blank text boxes for entering your criteria.
txtStartLat, txtEndLat, txtStartLong and txtEndLong are text boxes that are automatically populated through the”after update” event on the text boxes “txtPostCodeStart” and “txtPostCodeEnd” using simple RecordSet vba programming.

Private Sub caldistance_Click()
On Error GoTo Err_caldistance_Click

If Me.TxtPostCodeStart = “” Then
MsgBox (“Please enter a Start Post Code”)
Exit Sub
End If

If Me.TxtPostCodeEnd = “” Then
MsgBox (“Please enter an End Post Code”)
Exit Sub
End If

Distance = (Sin((Me.TxtEndLat * 3.14159265358979) / 180)) * (Sin((Me.TxtStartLat * _
3.14159265358979) / 180)) + (Cos((Me.TxtEndLat * 3.14159265358979) / 180)) *  _
((Cos((Me.TxtStartLat * 3.14159265358979) / 180))) * _
(Cos((Me.TxtStartLong – Me.TxtEndLong) * (3.14159265358979 / 180)))

Distance = 6371 * (Atn(-Distance / Sqr(-Distance * Distance + 1)) + 2 * Atn(1))

Me.TxTDistance = Distance

Exit_caldistance_Click:
Exit Sub

Err_caldistance_Click:
MsgBox Err.Description
Resume Exit_caldistance_Click
End Sub

Whilst we have demonstrated a simple form, the basic formula is there to be used in a host of different ways. We hope you will find this as useful as we have!
we have!

Written by Mick Jillions – Director of Mill House Data Solutions Ltd

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