[Proj] EPSG:27700

[Jose Gonçalves]

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


José


---------------------------------------------------------------------
José A. Gonçalves, University of Porto, Portugal





2010/1/27 Nick Hopton <nhopton at gmail.com>:
> I've just downloaded the latest version of Proj
> (proj446_win32_bin.zip) and notice that the parameters for EPSG:27700
> in the 'epsg' file are as follows:
>
> # OSGB 1936 / British National Grid
> <27700> +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601 +x_0=400000
> +y_0=-100000 +ellps=airy +units=m +no_defs  no_defs <>
>
> The problem is (I think!) that this doesn't take the datum shift into
> account, which leads to errors in the order of a hundred metres (112
> point-something metres at Greenwich). Shouldn't there be something
> like '+datum=OSGB36' in there? If I have this wrong please accept my
> apologies in advance, I'm not an expert.
>
> Regards,
> Nick.
>
> --
> Nick Hopton, Caversham, Reading, England.
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