Converts Ordnance Survey grid references to latitude and longitude, including transformation to WGS84 for GPS and googlemap compatibility
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This is a library that converts between British (and Irish) grid references and latitude and longitude co-ordinates. It is theoretically accurate to about 1m, but realistically you can expect precision of about 10m. Good for google map and GPS use, not recommended for surveying nuclear power plants. I don't suppose people often use grid references for that.


In Rails 3, add this to your Gemfile and run bundle install.

gem "osgb"

In Rails 2, add this to your environment.rb file.

config.gem "osgb"

Alternatively, you can install it as a plugin.

rails plugin install git://


Early days: activerecord interface hasn't settled down, some refactoring likely, bugs entirely possible. The basic algorithms are ancient and sound, though.


You don't need to make any explicit reference to the gem. It adds conversion and utility methods to the String class:

"SD28687846".is_gridref?                # -> true
"SD28687846".to_latlng                  # -> [54.196915, -3.094684]
"SD28687846".to_wgs84                   # -> [54.196915, -3.094684]
"SD28687846".to_osgb36                  # -> [54.196763, -3.093320]
"1.056789, 55.98978607".is_latlng?      # -> true

There is a Point class that's quite useful for geodetic calculations, and we provides some (tentative) help for your ActiveRecord classes:

class Checkpoint < ActiveRecord::Base
  has_gridref :lat => 'lat', 
              :lng => 'lng', 
              :gridref => 'gridref',
              :validation => false,
              :converstion => true

The :lat, :lng and :gridref keys should pass in the names of the relevant columns if they don't match these defaults.


By default we place our lat/long points on the WGS84 datum, since that's most relevant for online use. You can specify :osgb36 to keep them local:


which involves a bit less processing since that's the datum used by the grid. If you want to use some other datum you will need to define a Helmert transformation from OSGB36 to that ellipsoid.

Bugs and features

Github issues please, or for little things an email or github message is fine.

Author & Copyright

Copyright 2008-2011 Will at

Released under the same terms as Ruby