This library allows for conversion to and from British National Grid eastings and northings and WGS 84 (EPSG:4326) coordinates.
There are two methods that can be used to convert between BNG grid points and
WGS 84 coordinates, OSCoordinateForGridPoint and its inverse
OSGridPointForCoordinate, and OSCoordinateForGridPointUsing7Parameter and its
inverse, OSGridPointForCoordinateUsing7Parameter. The more concisely named
methods use OSTN02
to provide sub-1mm accuracy. The 7Parameter methods do not use OSTN02 so the
accuracy of the transformations can be as much as 7m out. However, depending on
your use case, or the data you're using, the 7 parameter functions may well be
the best solution.
OSGridPoint gridPoint = OSGridPointFromString(@"SU372155", NULL);
CLLocationCoordinate2D coordinate = OSCoordinateForGridPoint(gridPoint);
OSTransformation can be included in your project using Carthage. Just add:
github "OrdnanceSurvey/ostransformation-ios"
This library also contains a function to convert a WGS 84 altitude to the Orthometric
height as defined by OSGM02. However this should not be used with height values provided by Core Location's
CLLocationManager, as this already transforms the GPS height. Apple don't disclose
the source of their transformation, but from inspection, the value is relatively
close to the value you would get using OSGM02. If you wish to use this method,
you will need to include the OSGM02 file in the framework.
As with the OSTN02 data file, this library is licensed under the BSD license.
This project also depends on the Route-Me project, and on proj.4, so you must adhere to the licenses of those projects.