Update: November 2009
There have been almost 4,000 downloads to date. I’m working (slowly, I’ll admit) on adding height above mean sea level, 3GS compass support, coordinate emailing, tetrad display and more.
GridPoint GB is a simple, free iPhone utility.
It gives your current Ordnance Survey (OS) National Grid reference, and illustrates how to use this to locate yourself on an OS map.
It works only in Great Britain.
How is it useful?
- You’re out and about
- You have a paper OS map of the area
- You don’t have mobile phone reception
GridPoint GB could be useful when all three conditions are met. It makes it easy to locate yourself without a mobile signal—when Google Maps shows your position relative only to a featureless grey-and-white checkerboard.
How do I get it?
GridPoint GB requires the iPhone 3G or later, and is free on the App Store.
How do I use it?
Open GridPoint GB and tap ‘OK’ to 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 around 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 location and its accuracy are illustrated, as they would be seen on the grid of an OS map, by a white cross and a semi-transparent white circle.
To learn how to read an OS map, see the OS publication Map reading made easy.
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 3-digit one, just round off the rightmost 2 digits. For example, the 100m grid reference for the location shown in the screenshot is TQ 315 793.
How does it work?
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.
Confused? You don’t need to understand these details to make good use of GridPoint GB. But if you want to, OS provide an excellent introduction to coordinate systems.
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.