[gdal-dev] OSGB36 to WGS84 transformation

[Hermann Peifer]

On 2013-01-09 16:13, Johnny wrote:
>
> Are you aware of any way to batch process this in Linux to enable swift
> conversion of multiple references?
>

You'd need a script that maps the 2-letter grid cell identifiers to the 
respective offsets, then calculates the Easting and Northing values. I 
can send you an example off-list.

>> Please note that behind the scenes, these datum shift parameters are
>> applied: +towgs84=375,-111,431,0,0,0,0 (which might be what you want
>> .. or not).
>
> Sorry, I don't know much about cartography and datums, but guess these
> are the defaults by gdaltransform? Anyhow, it achieves what I want! :)

Indeed, these are the default values for a 3-parameter transformation 
(I'm using GDAL 1.9.2).

On page 33 of [1], OSGB gives the values for a 7-parameter 
transformation which should be more accurate in most cases. You have to 
revert the signs of the given values and use them like this: 
+towgs84=446.448,-125.157,542.06,0.1502,0.247,0.8421,-20.4894

And there is OSGB's grid shift file at [2] which leads to even more 
accurate results.

Hermann


[1] 
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf 


[2] 
http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html

>
>
> Regards,
>
> Johnny
>
> Footnotes:
> [1]  http://digimap.edina.ac.uk/webhelp/os/gazetteer_plus/grid_ref_conversion.htm
>