Sunday, 18 April 2010

OS Data part 3

The OS data is correct. I have checked a few points in polygons in the shp file in the OS projection and they are the same as a paper map, so somehow the transformation from OS to OSM is translating the result.

As an example a point on the transformed shp file is 53.76098, -0.48939 should be 53.76129, -0.49106.  As a bodge I'll add the translation to the OSM output and then test a few more places to see if the translation is consistent across the country.


Andrew said...

Have you allowed for the difference between the OS spheroid and WGS84?

Chris Hill said...

Thanks for your comment Andrew. I have applied a transform from the projection Ordnance Survey use to the WGS84 projection that OSM use. If you think there is another step or something else I need to do I would be very keen to know.

Ollie said...

Maybe the difference between OSGB36 and OSTN02? Although I think that difference is sub-metre.

If the difference is about 100m to the east in London then may well be due to the difference between the datums used by OSGB36 and WGS84.fe