GridPoint GB


Update: 29 June 2016

Version 2.0 is available in the App Store now. It works on iOS 9 and has some handy new features.

If you have a paper Ordnance Survey (OS) map, GridPoint GB makes it easy to locate yourself — even when you have no mobile reception.

GridPoint GB uses your iPhone’s GPS and compass to provide your current grid reference and heading, and illustrates how to use this to locate yourself on the map.

New in version 2, the app also:

Please note: this app works only in Britain (i.e. England, Scotland and Wales). It does not operate in other parts of the UK or British Isles — Northern Ireland or the Republic of Ireland — since the coordinate systems are not the same.

How do I get it?

GridPoint GB is free on the App Store.

How do I use it?

Open GridPoint GB and allow it access to your location. You may need to wait a few moments for your location to be determined. While the app is open your location will continue to be updated. Reported accuracy should improve to less than 10m if you are out in the open.

Your National Grid reference is given to the nearest metre, with 5-digit easting (E) and northing (N) values, as seen in the screenshot above. The accuracy of the reference, in metres, is given immediately below these (+/-). Your grid heading, and its accuracy, are also provided.

Your location and its accuracy are illustrated, as they would be seen on the grid of an OS map, by a white + and a semi-transparent white circle. Your heading is shown by the orientation of the iPhone symbol.

To learn how to read an OS map, see the OS’s map reading resources.

A note on grid references

Grid references are more commonly stated to the nearest 100m, with 3-digit eastings and northings. To convert a 5-digit easting or northing to a 4- or 3-digit one, just round off the rightmost 1 or 2 digits. For example, the 100m grid reference for the location shown in the screenshot is TQ 304 056.

How does it work?

OSTN02 logo

GridPoint GB converts the GPS WGS84 datum location reported by the iPhone into the OS standard OSGB36 datum.

It uses the OSTN02 National Grid Transformation, which OS makes freely available for this purpose, and which is accurate to within 1m. (The app also contains code implementing OSGM02, which converts WGS84 geoid height to height above mean sea level, but this is not currently exposed, since the iPhone does not report raw geoid height).

The app also calculates your heading relative to grid north, to display alongside the magnetic and true headings.

Grid north is calculated according to instructions in a 1983 OS publication and verified against a contemporary spreadsheet for coordinate calculations.

All conversion code is open-sourced on GitHub, and can also be used for batch conversions.

Confused? You don’t need to understand any of these details to make good use of GridPoint GB. But if you want to, OS provide an excellent introduction to coordinate systems.

Q & A

Does GridPoint GB use GPS (and run down my battery) in the background?

No. GridPoint GB does not run at all in the background. Simply quit the app (by pressing the home button) to terminate its use of GPS.

I see a message “Self-test failed”. What should I do?

At launch, the app runs around 100 tests on the integrity of its conversion data and routines. These tests should not fail, so if you see this message please send me an email (see below).


Please email me on george    mackerron com, or leave a review on the App Store. It’s great to hear how people are using the app.

Last updated: June 2016