[Proj] converting from UK ordinance survey to lat/lon

[Hugo Tyson]

Andrea Edwards [andrea.edwards at nulc.ac.uk] wrote:
> I am trying to use proj4 to convert from uk ordinance survey
> coordinates to latitude and longitude values. 
> 
> I found an example in the internet as so
> 
> C:\> proj.exe -I +init=world:bng +ellps=WGS84
> 353600 432900                        (input)
> 2d42'15.556"W   53d47'22.952"N       (output)
> 
> This does not work for me. I get this error
> projection initialization failure
> cause: no system list, errno: 2
> program abnormally terminated

Hi Andrea, 'fraid I don't know about that specific
initialisation issue.  I only ever spelled out the
args in full.

But a couple of comments/questions...

Are you converting to WGS84 Lat/Long or sticking to
Lat/Long relative to the Ordnance Survey datum (OSGB36)?
OSGB36 Lat/Long is the blue + marks on OS maps; they're
not WGS84.  If OSGB36 you'll be OK with proj.  If WGS84
you'll need to use cs2cs (which is part of the proj suite
of tools) instead, because it does datum changes as well
as projections.

I was doing this the other way - WGS84LatLong to OSGB36
British National Grid.  I found that I needed to add a datum
to proj, so that cs2cs can do the datum change.  Details are
in the archive of this list.

If you make that change and rebuild proj et al, you should
be able to do your conversion (to WGS84 datum) with

cs2cs +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601
      +x_0=400000 +y_0=-100000 +units=m
      +ellps=airy +datum=OSGB36
      +to
      +proj=latlong +ellps=WGS84 +datum=WGS84

(which doesn't give the same answers as your web example)
Hope this helps,

	- Huge

-- 
Hugo Tyson, Cambridge Positioning Systems Ltd
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