Convert Six Figure OS Reference into OS Grid Reference

Subject: Programming

Description: How to convert an Ordnance Survey Map Reference into a Grid Reference

Posted by David Caldwell on 22/02/14 at 19:52

How to Convert OS Eastings and Northings into an OS Grid Reference

For example how to turn 432648, 390657 into SK39

I needed to append an Ordnance Survey grid reference to some data in a Microsoft Access database, to do this programmatically I needed to get back to basics and learn how OS grid references work.

The UK falls within five map grids each one measuring 500km by 500km, these are referenced as H, N, O, S and T.

Each 500km grid is then divided into 25 100km by 100km squares, these are referenced by two letters.  The first character denotes the larger 500km square and the second the smaller 100km square.  For some reason the letter ‘I’ is omitted. This is possibly because the map system predated the letter I's discovery in 1967 by Sidney Poitier.

Now we have a framework we can start to write some code for a module called Grid which takes a pair of easting and northings as an argument and returns a grid reference.

I am not a native VB programmer so apologies if my code is a bit clunky.

Public Function Grid(x, y) Dim eastings As String Dim northings As String Dim firstref As String Dim secondref As String Dim intx As Long Dim inty As Long Dim firstx As String Dim firsty As String eastings = x northings = y If eastings > 500000 Then eastings = eastings - 500000 End If If northings > 1000000 Then northings = northings - 1000000 End If If northings > 500000 Then northings = northings - 500000 End If firstx = Mid(CStr(eastings), 2, 1) firsty = Mid(CStr(northings), 2, 1) intx = x inty = y If (intx < 500000 And inty < 500000) Then firstref = "S" End If If (intx > 500000 And inty < 500000) Then firstref = "T" End If If (intx < 500000 And inty > 500000) Then firstref = "N" End If If (intx > 500000 And inty > 500000) Then firstref = "O" End If If (inty > 1000000) Then firstref = "H" End If If eastings < 100000 And northings > 400000 Then secondref = "A" End If If eastings > 400000 And northings > 400000 Then secondref = "E" End If If eastings > 400000 And northings < 100000 Then secondref = "Z" End If If eastings < 100000 And northings < 100000 Then secondref = "V" End If If eastings < 500000 And eastings > 400000 And northings < 200000 And northings > 100000 Then secondref = "U" End If If eastings < 100000 And northings > 300000 And northings < 400000 Then secondref = "F" End If If eastings < 100000 And northings > 200000 And northings < 300000 Then secondref = "L" End If If eastings < 100000 And northings > 100000 And northings < 200000 Then secondref = "Q" End If If northings > 400000 And eastings < 200000 And eastings > 100000 Then secondref = "B" End If If northings > 400000 And eastings < 300000 And eastings > 200000 Then secondref = "C" End If If northings > 400000 And eastings < 400000 And eastings > 300000 Then secondref = "D" End If If northings < 100000 And eastings < 200000 And eastings > 100000 Then secondref = "W" End If If northings < 100000 And eastings < 300000 And eastings > 200000 Then secondref = "X" End If If northings < 100000 And eastings < 400000 And eastings > 300000 Then secondref = "Y" End If If eastings > 400000 And northings > 300000 And northings < 400000 Then secondref = "K" End If If eastings > 400000 And northings > 200000 And northings < 300000 Then secondref = "P" End If If eastings > 400000 And northings > 100000 And northings < 200000 Then secondref = "U" End If If eastings > 100000 And eastings < 200000 And northings < 400000 And northings > 300000 Then secondref = "G" End If If eastings > 200000 And eastings < 300000 And northings < 400000 And northings > 300000 Then secondref = "H" End If If eastings > 300000 And eastings < 400000 And northings < 400000 And northings > 300000 Then secondref = "J" End If If eastings > 100000 And eastings < 200000 And northings < 300000 And northings > 200000 Then secondref = "M" End If If eastings > 200000 And eastings < 300000 And northings < 300000 And northings > 200000 Then secondref = "N" End If If eastings > 300000 And eastings < 400000 And northings < 300000 And northings > 200000 Then secondref = "O" End If If eastings > 100000 And eastings < 200000 And northings < 200000 And northings > 100000 Then secondref = "R" End If If eastings > 200000 And eastings < 300000 And northings < 200000 And northings > 100000 Then secondref = "S" End If If eastings > 300000 And eastings < 400000 And northings < 200000 And northings > 100000 Then secondref = "T" End If Grid = firstref + secondref + firstx + firsty End Function PHP Implimentation // calculate OS grid number $easting = $lat; $northing = $lng; // process the 500km squares If ($easting < 500000 And $northing < 500000){ $firstref = "S"; } If ($easting > 500000 And $northing < 500000){ $firstref = "T"; } If ($easting < 500000 And $northing > 500000){ $firstref = "N"; } If ($easting > 500000 And $northing > 500000){ $firstref = "O"; } If ($easting > 1000000){ $firstref = "H"; } // process the 100km squares if($easting > 500000) { $easting = $easting - 500000; } if($northing > 1000000) { $northing = $northing - 1000000; } if($northing > 500000) { $northing = $northing - 500000; } $gridref =array("00"=>"V","01"=>"W","02"=>"X","03"=>"Y","04"=>"Z","10"=>"Q","11"=>"R","12"=>"S","13"=>"T","14"=>"U","20"=>"L","21"=>"M","22"=>"N","23"=>"O","24"=>"P","30"=>"F","31"=>"G","32"=>"H","33"=>"J","34"=>"K","40"=>"A","41"=>"B","42"=>"C","43"=>"D","44"=>"E"); $x_string = strval($easting); $y_string = strval($northing); if (strlen($x_string) < 6){ $start_x = '0'; $numbers_x = '0'; } else{ $start_x = substr($x_string, 0,1); $numbers_x = substr($x_string, 1,1); } if (strlen($y_string) < 6){ $start_y = '0'; $numbers_y = '0'; } else{ $start_y = substr($y_string, 0,1); $numbers_y = substr($y_string, 1,1); } $key = $start_y.$start_x; $grid = $gridref[$key]; $gridtable = strtolower($firstref.$grid); $osgrid = $firstref.$grid.$numbers_x.$numbers_y;

Keywords: OS Grid, Module, VB, Map Reference, Microsoft Access

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