[gdal-dev] Help: problem reprojecting a raster from British National Grid to Google Maps

[Oliver Smith]

Hi,

I've got a data set consisting of a bunch of tiled rasters, in a
British National Grid (EPSG:27700) projection.  I'm trying to
re-project them to the Google Maps projection (see below for the
proj.4 string I'm using to describe the projection).  The commands I'm
using have previously worked fine for images from the Blue Marble data
set, and from a Portuguese street map data set (in a Lambert Conic
projection), but the British images are being offset to the south-east
by about 200m when I re-project.

The same British source data re-projects fine into EPSG:4326 for
overlaying on Google Earth, so I'm somewhat confused about where my
errors are creeping in!

I'm using GDAL 1.5.2, my Google Maps projection string is:
+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
+y_0=0 +k=1.0 +units=m +nadgrids=null +no_defs

If anyone can offer any tips, it'd be most appreciated.

Oliver