[Proj] EPSG:27700

[Nick Hopton]

Jose wrote:

> I think datum definition is not included in the epsg file when there
> are different sets of parameters or forms of transformation from the
> local datum to WGS84. +datum=OSGB36 corresponds to a given set of
> parameters, which may not be the most appropriate for some
> applications. You can also do the datum transformation using NTv2
> grids instead of Bursa-Wolf transformation (see for example
> http://lists.maptools.org/pipermail/proj/2006-November/002658.html).
> Users are free to combine projection 27700 with their own datum
> transformation. You can have for example:
>
> cs2cs +init=4326 +to +init=27700 +datum=OSGB36
> or
> cs2cs +init=4326 +to +init=27700 +towgs84=.....
> or
> cs2cs +init=4326 +to +init=27700 +nadgrids=c:/england-wales.gsb
>
> Regards

Thanks for this. I think that EPSG: 27700 is specific to the Ordnance
Survey of Great Britain and is meant to stand alone. People using Proj
to project or reproject from or to 27700 will be getting errors in
excess of a hundred metres, which is a large enough error to cause
problems, but not necessarily large enough to be noticed at first
sight.

Regards,
Nick.