[gdal-dev] Guidance sought on using gdal-translate

[Frank Warmerdam]

On Fri, Aug 15, 2008 at 8:41 AM, Christopher Hunt
<huntc at internode.on.net> wrote:
> Hi again Frank et. al,
>
> I've made some progress. The aerial image provider now tells me that it is
> projected as OSGB 1936 which I believe is EPSG:27700. Thus if I use a
> command like:
>
> gdal_translate -a_srs EPSG:27700 -a_ullr ....
>
> then I should be able to create my GeoTIFF.
>
> Unfortunately the image provider hasn't provided the top left/bottom right
> coords so I'm going to Google Earth to approximate them (yes, ugly I know).
> I'm hopeful that the actual top left/bottom right will ultimately be
> provided to me, but meanwhile I want to ensure that the a_ullr parameters I
> provide will be in metres i.e. eastings and northings from the OSGB 1936
> origin. Is this correct?

Chris,

I believe EPSG:27700 is British National Grid which is a projected
coordinate system based on the OSGB 1936 datum.  So you should
ensure the provider means the dataset is actually in BNG, in addition
to being based on OSGB 1936.

If you can get BNG coordinates then that is great.  If you can only
get WGS84 or OSGB1936 lat/long coordinates then you can
convert these to BNG using a program like cs2cs though that is
a bit tricky.

Best regards,

-- 
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I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
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