[Proj] towgs84 values for EPSG:27700

[Hamish]

Janne wrote:
> Hello,
> 
> Spatial Reference Org
> 
> http://spatialreference.org/ref/epsg/27700/
> http://spatialreference.org/ref/epsg/27700/proj4/
> 
> gives:
> 
> +proj=tmerc
> +lat_0=49
> +lon_0=-2
> +k=0.9996012717
> +x_0=400000
> +y_0=-100000
> +ellps=airy
> +datum=OSGB36
> +units=m
> +no_defs 
> 
> which is the OSGB 1936 / British National Grid. The
> OSGB36 datum is defined differently in both
> applications. There are several different definitions
> for the OSGB36 and more are added as people need
> definitions for different purpose.
> 
> So the reason for the difference is that the OSGB36
> datum is not any standard value.


see http://trac.osgeo.org/proj/ticket/122

(7 term is by not "by definition better" than 3 term +towgs84;
for any given datum there may be a dozen or more transform terms
which are valid/used in differing particular regions; sometimes
you want to match what was used in an earlier dataset rather than
what is today considered ideal; the official EPSG.org db does not
define datum transform terms, those are added by end users (pj4);
if the proj4 epsg code defines +towgs84 terms then tacking a
+nadgrids file at the end creates a conflict which needs string
substitution to fix, but because +datum is now removed from
proj4's copy of epsg you can't automate that process from a
lookup table; qgis is patching around this on a per-datum basis,
grass gis's datum support is more heavily dependent on proj/gdal
and is just plain broken by this; ...)


thanks,
Hamish