[Proj] OSGB36 to wgs84 decimal

Frank Warmerdam warmerdam at pobox.com
Tue Mar 21 10:41:27 EST 2006

Philipp H. Mohr wrote:
> Hello,
> 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 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.

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?

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGF, http://osgeo.org

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