[Proj] EPSG:27700

Nick Hopton nhopton at gmail.com
Wed Jan 27 14:11:55 EST 2010

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.


Nick Hopton, Caversham, Reading, England.

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