Geography::NationalGrid::GB(3) Module to convert British National Grid references to/from Latitude and Longitude


You should create the object using the Geography::NationalGrid factory class, but you still need to know the object interface, given below.

my $point1 = new Geography::NationalGrid::GB(
GridReference => 'TQ 289816',
my $point2 = new Geography::NationalGrid::GB(
Latitude => 51.783,
Longitude => 0
print "Point 1 is " . $point->latitude . " degrees north\n";


Once created, the object allows you to retrieve information about the point that the object represents. For example you can create an object using a grid reference and the retrieve the latitude and longitude.


These are the options accepted in the constructor. You MUST provide either a GridReference or Latitude and Longitude, or Easting and Northing (the origin for these is the usual location of SV 000000).
Default is 'NATGRID', the familiar National Grid as found on Ordnance Survey maps. Other projections recognized are 'IRNATGRID', 'UTM29', 'UTM30', and 'UTM31', which stand for the Irish National Grid, and the UTM29 to 31 zones. This argument is a string.

NOTE: if you use a projection other than the default then the results for the gridReference() method will be wrong, so the method will return undef. However, you can use the northing() and easting() results instead to find the location in the desired projection.

A grid reference string composed of the following: a 2-letter 100km square identifier; an even number of digits, from 2 to 10, depending on required accuracy; an optional quadrant identifier, one of NW, NE, SW, SE. A standard 6-figure reference such as 'TQ 289816' gives 100m accuracy. Case and whitespace is ignored here. Quadrant is currently ignored.
The latitude of the point. Actually should be the latitude using the spheroid related to the grid projection but for most purposes the difference is not too great. Specify the amount in any of these ways: as a decimal number of degrees, a reference to an array of three values (i.e. [ $degrees, $minutes, $seconds ]), or as a string of the form '52d 13m 12s'. North is positive degrees, south is negative degrees.
As for latitude, except that east is positive degrees, west is negative degrees.
The number of metres east of the grid origin, using grid east.
The number of metres north of the grid origin, using grid north.
If true this inhibits warnings about using points outside the area covered by Ordnance Survey maps. Default is false.
The value of this option is a hash-reference, which you can fill with whatever you want - typical usage might be to specify Userdata => { Name => 'Greenwich Observatory' } but add whatever you want. Access using the data() method.


Most of these methods take no arguments. Some are inherited from Geography::NationalGrid
Returns the latitude of the point in a floating point number of degrees, north being positive.
As latitude, but east is positive degrees.
gridReference( [ RESOLUTION ] )
Returns the grid reference of the point in standard format. The default resolution is 100m, or if you used a grid reference in the constructor then the default resolution is the resolution of that reference. You can explicitly set the resolution to 1, 10, 100, 1000, or 10000 metres.
How many metres east of the origin the point is. The precision of this value depends on how it was derived, but is truncated to an integer number of metres. For example if the object was created from a 6 figure grid reference the easting only has precision to 100m.
How many metres north of the origin the point is. The precision of this value depends on how it was derived, but is truncated to an integer number of metres.
deg2string( DEGREES )
Given a floating point number of degrees, returns a string of the form '51d 38m 34.34s'. Intended for formatting, like: $self->deg2string( $self->latitude );
Returns the item from the Userdata hash whose key is the PARAMETER_NAME.


The routines used in this code may not give you completely accurate results for various mathematical and theoretical reasons. In tests the results appeared to be correct, but it may be that under certain conditions the output could be highly inaccurate. It is likely that output accuracy decreases further from the datum, and behaviour is probably divergent outside the intended area of the grid.

This module has been coded in good faith but it may still get things wrong. Hence, it is recommended that this module is used for preliminary calculations only, and that it is NOT used under any circumstance where its lack of accuracy could cause any harm, loss or other problems of any kind. Beware!

That said, the 2 worked examples provided by the Ordnance Survey yield the correct results with this module. Further tests are needed, and comparison with actual Landranger or larger-scale OS maps.


Equations for converting co-ordinate systems appear in the guide at - entitled ``A guide to coordinate systems in Great Britain: A primer on coordinate system concepts, including full information on GPS and Ordnance Survey coordinate systems.''

National Grid letter-pairs checked at

ISO 3166 Country codes checked against

Conversions compared with software from and online services


Copyright (c) 2002 P Kent. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

$Revision: 1.3 $