[Proj] OSGB36 to wgs84 decimal

Philipp H. Mohr phm4 at kent.ac.uk
Tue Mar 21 10:58:47 EST 2006


thank you very much for your reply.

> > I am new to proj.4 and cs2cs and I am utterly confused by all the options.
> > I would like to convert from British National Grid OSGB36 into wgs84. I am
> > aware that this topic has been covered already, but not the output I am
> > after.
> >
> > I would like to convert, e.g.: 158427.13145307 615597.12638647
> >
> > into e.g.: 51.29647 1.06660333333
> Can you be more specific about what error you are seeing?  I get a small
> difference but I'm not sure if that is what you are concerned about.

I haven't converted one to the other, this was just meant as an example
for the format I am interested in.

> I would also encourage you to be more specific about axis direction.
> I first assumed you expected the output near 51E, 1N and fairly quickly
> realized that was crazy for the UK national grid.  But of course you
> meant 1E, 51N.

Ok, I agree, wasn't sure which way around is the standard - now I know :-)

> But even more perversely you reversed the easting and northing which
> isn't so obvious from the raw values.  I only determined this by
> trying them the other way around.   I get:
> cs2cs +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 +to +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +nodefs
> 615597.12638647 158427.13145307                   (input)
> 1d5'25.031"E    51d17'2.982"N 44.150              (output)
> This is reasonably close to your target location but still off by
> a more than a second.  (BTW your location in DMS is 1d3'59.772"E, 51d17'47.292"N)
> Is this the difference you are concerned about?

I would like to get an accurate conversion which outputs the wgs84
co-ordinates in decimal degree and not in degree minutes and seconds.

Thank you very much,

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