[Proj] OSGB36

Paul Kelly paul-grass at stjohnspoint.co.uk
Thu Jan 12 12:19:27 EST 2006

Hello Roger

On Thu, 12 Jan 2006, Roger Oberholtzer wrote:

> This time, I am testing OSGB36 (UK). My current method uses geotrans. I
> am using a 7 parameter transform (in geotrans-speak). When I convert
> WGS84 lat/longs into OSGB36 X/Y, the difference from the accepted
> location is less than 10 meters (which is an issue for a future
> posting). So, I thought I would see how proj4 compares. Once again, my
> brain is nowhere in sight. I get location errors that are much greater.
> Usually over 90 meters.
> I am trying this:
> cs2cs +proj=latlong +ellps=WGS84 +to +proj=tmerc +lat_0=49 +lon_0=-2
> +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs
> These settings are based on the esri file and look reasonable compared
> to the values I use in geotrans.

You need to do a datum transformation (WGS84 to OSGB36)
With the following command line
cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +nodefs +to +proj=tmerc 
+lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy 
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m 
I get these results
620286.26       309466.38
for your test point. Much closer.

That parameter set is used in GRASS and the comment with it indicates an 
accuracy of 5 metres. For anything better you're likely to need to use 
some of the gridshifting data available at http://www.gps.gov.uk/ (not yet 
in a suitable format for use with PROJ.4)


> A sample set of points:
> WGS84 (somewhere near Norwich):
> 	lat=52.63820000
> 	lng=1.25423000
> I should get (UK TRL say):
> 	N=620282.88  E=309469.17
> But with the command above I get:
> 	N=620164.87  E=309430.14
> With geotrans (C API), I get:
> 	N=620287.247 E=309465.242
> I really do try to figure this out before I post. But I am stumped.
> Again...
> --
> Roger Oberholtzer
> OPQ Systems AB
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