[Proj] OSGB36

Roger Oberholtzer roger at opq.se
Thu Jan 12 11:58:47 EST 2006

Here I go again :)

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.

A sample set of points:

WGS84 (somewhere near Norwich):



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.

Roger Oberholtzer
OPQ Systems AB

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