[gdal-dev] Reprojecting rasters

[Rob]

Hi

[Apols for the X-post]

I'm trying to reproject a set of rasters in order to improve performance on
my application.

I have a set of rasters in OSGB (EPSG:27700) that I want to pre-process so
that they are in WGS84 (EPSG:4326) within GeoServer, rather than relying on
GeoServer to do the reprojection on the fly.

I have been reprojecting the rasters using the following command


gdalwarp -of GTiff -s_srs "EPSG:27700" -t_srs "EPSG:4326" -co "TILED=YES"
-co "COMPRESS=LZW" -co "TFW=YES" OLDNAME.TIF NEWNAME.TIF


I have used the ImageMosaic datastore to load these into GeoServer and can
view them.  So far, so good.

However, there seems to be large gaps between the 'tiles' so that roads no
longer join up and forests have big white gaps within them.

>From checking the rasters and the resultant world files, I've noticed a few
things.

1) The image sizes are not constant.  All of my original tiles are the same
size.  It has raised an interesting question of whether the meridian line
runs parallel to an OSGB Y-axis.
2) The worldfiles also contain some curious values.  I understand that the
1st and 4th parameters in a worldfile are the real world distance of a
single pixel, but they seem to have the same value in despite the fact that
the image size changes from being square to being rectangular.

Whether those things mean anything, or are perfectly legitimate when
reprojecting, I don't know, I just thought I should mention them.

Has anybody got any advice on how to reproject a full set of rasters, or
what I have done wrong?

Thanks

Rob
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