Question

Anyone know how to convert latitude, longitude in degrees to define a BBOX where SRS=EPSG:27700?

I am trying to call a WMS service with a URL like following (not a real link): http://mysecretmappingserver.com/wms?user=myuser&pwd=mypassword&VERSION=1.1.1&REQUEST=GetMap&LAYERS=ap25cm&STYLES=&SRS=EPSG:27700&BBOX=229096,231675,229296,231875&width=400&height=400

~~Any language would be fine; C# preferable.~~

Spacedman has been trying to help me, but I can t seem to get Proj4Net to work for me - all me, I m sure - but if someone knows either Proj4Net or the math involved, that might be better...

Answer 1

You need an interface to the PROJ.4 projections library in your language of choice. In R, for example, its in the rgdal package:

Here s some points (1,1 to 2,2 degrees) in epsg:4326:

> pts
SpatialPoints:
     coords.x1 coords.x2
[1,]         1         1
[2,]         2         2
Coordinate Reference System (CRS) arguments: +init=epsg:4326

and voila:

> spTransform(pts,CRS("+init=epsg:27700"))
SpatialPoints:
     coords.x1 coords.x2
[1,]  734005.9  -5416918
[2,]  845270.7  -5305999
Coordinate Reference System (CRS) arguments: +init=epsg:27700

Proj.4 docs here:

http://trac.osgeo.org/proj/

Since this is OSGB, probably a better example would be in the UK: here s a point in central london:

> pts = SpatialPoints(cbind(-0.109863,51.460852),proj4string=CRS("+init=epsg:4326"))
> spTransform(pts,CRS("+init=epsg:27700"))SpatialPoints:
     coords.x1 coords.x2
[1,]  531407.1  175235.8
Coordinate Reference System (CRS) arguments: +init=epsg:27700
+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000
+y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894

Answer 2

I recommend ogr2ogr, which among other things can convert between projections. I have it installed on my Mac, and there are binding e.g. to Python and many other languages. You can also use it on the commandline. Homepage is http://www.gdal.org/ogr2ogr.html

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