[Talk-GB] OSM UK's first tile layer

[Adrian]

The Ordnance Survey provides a transformation between OSGB36 and ETRS. It is described on this page https://www.ordnancesurvey.co.uk/gps/transformation/ and on the pages linked from there. The transformation is definitive. In other words, OSGB36 is redefined as being what you get when you apply the transformation to sets of ETRS co-ordinates. This must mean that if you compare an OS 1:1250 National Grid plan, with an older version of the plan from the era of OSTN02, features may have shifted slightly.

The transformation involves a mathematical transformation, and an adjustment based on a look-up table, to make the result match the errors in the old triangulation system. The OS provides applications to do the transformation, both ways, for a range of platforms. It also provides source code, the look-up table, and details of the mathematical transformation.

JOSM handles projections using proj. If you want to know what JOSM does with EPSG:27700, you need to know how it is defined in proj. The source code of JOSM includes the OSTN02 look-up table (15MB), but it can't be in the jar (also 15MB), so I don't know how that works.

Rob asked about position errors from the Helmert transformation without a look-up table. Here are some examples.
Larger errors
Place             error, m
St Kilda           4.9
Scilly             4.7
Lizard Point       4.1
Butt of Lewis      3.2
King's Lynn        2.7
Mallaig            2.6
Flamborough Head   2.4
Colchester         2.4
Plymouth           2.4
Nottingham         2.3
Anglesey           2.1
Northampton        2.0
North Foreland     1.9
Isle of Man S      1.9
Carmarthen         1.9
Smaller errors
St Catherine's Pt  1.4
Carlisle           0.8
Edinburgh          0.6
Aberdeen           1.8
Thurso             1.6
Orkney             1.0
Foula (Shetland)   1.2
The errors are particularly small near Bristol, Edinburgh and Fair Isle. They exceed 2m in South Devon, Cornwall, East Anglia, Nottinghamshire, Lincolnshire, the East Riding of Yorkshire, Pembrokeshire, Anglesey and Western Scotland.

+1 for referencing GB to ETRS.