I have an interpolated raster image with EPSG:27700 (British National Grid), after I warp it using gdalwarp into EPSG:3857 (WGS84 Web Mercator (Auxiliary Sphere)):
gdalwarp -s_srs EPSG:27700 -t_srs EPSG:3857 input.tif output.tif
which works fine (GDAL 1.10.1), (including a little bit of twisting (black cells) in the output one which is understandable as I guess there must be a little bit of distortion).
Then, if I create a Web Map Service (wms) in Geoserver using the output file as datastore, when I display the wms in a web page, by using OpenLayers and Google Maps as background, the image is not perfectly located, as it is misplaced about 90-100 meters LEFT where it should be in the British National Grid projection.
My question is whether gdalwarp does not the right transformation and/or OpenLayers does its best and does not adjust it, or it is definitely impossible to transform 27700 into 3857 accurately.