[Proj] OSGB36

Oscar van Vlijmen ovv at hetnet.nl
Fri Jan 13 08:19:40 EST 2006

```> From: Paul Kelly <paul-grass-stjohnspoint.co.uk>
> Subject: Re: [Proj] OSGB36
> 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).
>> 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

> 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
> +no_defs
> I get these results
> 620286.26       309466.38
> for your test point. Much closer.

I can confirm the test results from mr. Kelly.
A more verbose description:
Transformation from WGS84 to OSGB36:
lat=52.63820; lon=1.25423; h=0;
This gives:
lat=52.637798, lon=1.256045, h=-45.3413
Transformation parameters:
dx=-446.448; dy=125.157; dz=-542.060;
rx=-0.1502; ry=-0.2470; rz=-0.8421; ds=20.4894;
Position vector rotation, approximate transform (like PROJ.4 does).
Transverse Mercator on Airy 1830 ellipsoid:
with: lat0=49.0; lon0=-2.0; x0=400000; y0=-100000; k0=0.9996012717;
gives: x=620286.2515, y=309466.3864

Source of the parameters:
"A guide to coordinate systems in Great Britain", Ordnance Survey, v1.4 May
2002

Differences with Ordance Survey results must be attributed to lack of
gridinterpolation.

Geotrans has inaccurate transformation parameters in the file 7_param.dat
Note that these are coordinate frame parameters.
With these parameters I arrive with my code at
x = 620287.2473, y = 309465.2385
(mr. Oberholtzer mentioned N=620287.247 E=309465.242)

By the way, in my previous email with subject "Re: [Proj] proj and Dutch RD"
I forgot to mention that the datum transformation parameters are coordinate
frame parameters, so the signs of the r's should be reversed if you want to
use the parameters for a position vector transform like PROJ.4's.

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