This is a C implementation of Ordnance Survey’s definitive transformation model OSTN02 and Geoid model OSGM02, with tests.

• It powers version 2 of my GridPoint GB iOS app.

• It includes a simple command-line tool.

• It also builds with emscripten and now comes with (as yet undocumented) embind bindings, so you can use it from JavaScript too.

Licence

I’m releasing the code under the MIT licence (note, however, that OSTN02 and OSGM02 are trademarks of Ordnance Survey, and that the OSTN02 and OSGM02 data are © Crown copyright 2002. All rights reserved).

Using the command-line tool

Thanks to emscripten you can now use the command line tool here in your browser.

It works like this:

dyn204:OSTN02 George$ ostn02c help

OSTN02C -- https://github.com/jawj/OSTN02C -- Built Feb 14 2012 11:53:48 (double precision)

This tool converts coordinates between ETRS89 (WGS84, GPS) lat/lon/elevation and OSGB36 easting/northing/elevation.
Conversions make use of Ordnance Survey's OSTN02 and OSGM02 transformations, and should thus be accurate to within 1m.

Usage: ostn02c gps-to-grid [lat lon elevation] converts ETRS89 to OSGB36
       ostn02c grid-to-gps [easting northing elevation] converts OSGB36 to ETRS89
       ostn02c list-geoids lists the geoid datum flags, names and regions
       ostn02c test checks embedded data integrity and runs conversion tests with known coordinates
       ostn02c help displays this message

The conversion commands gps-to-grid and grid-to-gps can be used in two ways:
* Given a set of coordinates as command-line arguments, they will convert this set with user-friendly output
* Given no command-line arguments, they will convert batches of coordinates, reading from STDIN and writing to STDOUT

In the batch conversion case:
* Input rows must have 3 columns -- lat or easting, lon or northing, elevation -- with any reasonable separator (,;:| \t)
* Output rows have 4 columns -- easting or lat, northing or lon, elevation, geoid datum flag -- separated with tabs
* In case of out-of-range input coordinates, all output columns will be zero
* Malformatted input terminates processing and results in a non-zero exit code

Software copyright (c) George MacKerron 2012 (http://mackerron.com)
Released under the MIT licence (http://www.opensource.org/licenses/mit-license.php)

OSTN02 and OSGM02 are trademarks of Ordnance Survey
Embedded OSTN02 and OSGM02 data (c) Crown copyright 2002. All rights reserved.


dyn204:OSTN02 George$ ostn02c gps-to-grid 51.0 -0.5 100.0

ETRS89 in  lat:   51.000000, lon:   -0.500000, elevation:  100.000
OSGB36 out   E:  505349.663,   N:  123360.984, elevation:   54.635 (UK mainland / Newlyn)


dyn204:OSTN02 George$ ostn02c grid-to-gps 505349.663 123360.984 54.635

OSGB36 in    E:  505349.663,   N:  123360.984, elevation:   54.635 (UK mainland / Newlyn)
ETRS89 out lat:   51.000000, lon:   -0.500000, elevation:  100.000


dyn204:OSTN02 George$ echo "51.0, -0.5, 100.0
> 51.5, -1.0, 50.0" | ostn02c gps-to-grid
505349.663	123360.984	54.635	1
469509.184	178369.364	3.360	1

dyn204:OSTN02 George$ echo $?
0


dyn204:OSTN02 George$ echo "51.0, -0.5, 100.0
> 20.0 40.0 0.0
> an invalid line
> 51.5, -1.0, 50.0" | ostn02c gps-to-grid
505349.663	123360.984	54.635	1
0.000	0.000	0.000	0

dyn204:OSTN02 George$ echo $?
1


dyn204:OSTN02 George$ ostn02c list-geoids

Flag  Datum          Region
   0  N/A            Outside model boundary
   1  Newlyn         UK mainland
   2  St Marys       Scilly Isles
   3  Douglas02      Isle of Man
   4  Stornoway      Outer Hebrides
   5  St Kilda       St Kilda
   6  Lerwick        Shetland Isles
   7  Newlyn         Orkney Isles
   8  Fair Isle      Fair Isle
   9  Flannan Isles  Flannan Isles
  10  North Rona     North Rona
  11  Sule Skerry    Sule Skerry
  12  Foula          Foula
  13  Malin Head     Republic of Ireland
  14  Belfast        Northern Ireland
  
  
dyn204:OSTN02 George$ ostn02c test

Original CRC32 (data):  790474494
Computed CRC32 (data):  790474494

Original CRC32 (index): 244629328
Computed CRC32 (index): 244629328

ETRS89 actual   lat:   53.779110, lon:   -3.040455, elevation:   64.940
OSGB36 actual     E:  331534.552,   N:  431920.792, elevation:   12.636 (UK mainland / Newlyn)
ETRS89 computed lat:   53.779110, lon:   -3.040455, elevation:   64.940
OSGB36 computed   E:  331534.552,   N:  431920.792, elevation:   12.636 (UK mainland / Newlyn)

ETRS89 actual   lat:   51.427547, lon:   -2.544076, elevation:  104.018
OSGB36 actual     E:  362269.979,   N:  169978.688, elevation:   54.467 (UK mainland / Newlyn)
ETRS89 computed lat:   51.427547, lon:   -2.544076, elevation:  104.018
OSGB36 computed   E:  362269.979,   N:  169978.688, elevation:   54.467 (UK mainland / Newlyn)

[...]

ETRS89 actual   lat:  -51.000000, lon:  180.000000, elevation:  100.000
OSGB36 actual     E:       0.000,   N:       0.000, elevation:    0.000 (Outside model boundary / N/A)
OSGB36 computed   E:       0.000,   N:       0.000, elevation:    0.000 (Outside model boundary / N/A)

ETRS89 actual   lat:    0.000000, lon:    0.000000, elevation:  100.000
OSGB36 actual     E:       0.000,   N:       0.000, elevation:    0.000 (Outside model boundary / N/A)
OSGB36 computed   E:       0.000,   N:       0.000, elevation:    0.000 (Outside model boundary / N/A)

92 tests; 92 passed; 0 failed

Installing the native command-line tool

For Mac

You can download a binary for Mac OS X 10.4+, Intel 32/64-bit. You’ll need to uncompress, chmod +x and mv this somewhere appropriate.

Alternatively, use Homebrew. In Terminal.app, type:

brew install https://jawj.github.io/OSTN02C/ostn02c.rb

From source

Otherwise, with the relevant build tools installed — Xcode on the Mac, sudo aptitude install build-essential on Debian/Ubuntu, etc. — download or clone the source to your machine, and compile like so:

gcc OSTN02/*.c -std=gnu99 -D_GNU_SOURCE -lm -Wall -Wno-missing-braces -O2 -o ostn02c

To compile with long double precision arithmetic (not recommended, since the outputs are the same to within 1mm), add the option -DUSE_LONG. To suppress fancy formatting (bold/inverse/underlined) in the tool’s output, add the option -DUNFANCY_OUTPUT.

Output buffering

If you’re using the command line tool from another script (e.g. from Ruby or Python), alternately writing coordinates to its STDIN and reading the converted results from its STDOUT, you’ll need to ensure that your I/O isn’t being buffered.

To help with this, the tool checks for an environment variable named OSTN02C_LINE_BUFFERED, and does buffering at line level only (i.e. output is flushed after every newline character) if it’s set.

As an example, you can use this snippet to convert WGS84 to OSGB36 in Ruby:

@ostn02 = IO.popen([{'OSTN02C_LINE_BUFFERED' => 'Y'}, 'ostn02c', 'gps-to-grid'], 'r+')
@ostn02.sync = true  # this is actually the default on my system
def ll_to_en(lat, lon, elev)
  @ostn02.puts "#{lat} #{lon} #{elev}"
  osgb = @ostn02.gets.split("\t")
  datum = osgb[3].to_i
  return nil unless datum > 0
  osgb[0..2].map(&:to_f)
end