[gdal-dev] Using a gdal generated tiff to generate DEM tiles - losing registration

[Iain]

I'm building a set of 'tiles' that actually just contain dem data to drive a
3d geograpic tool on android (using opengl es2.0)

primarily for the uk (cos that's where I am) but hopefully in a way that
works anywhere.

I've picked up the srtm data and used gdal translate and gdawarp as
described in the  http://wiki.openstreetmap.org/wiki/HikingBikingMaps
openstreetmao wiki here , and used these to add height colouring and contour
lines to mapnik generated tiles and this all works well and appears to be
OK.

To get the 'height' tiles I'm using the same giant tiff that I use as input
to hill shade and colour-relief to a little home brew app that uses the gdal
library to fetch out a tilesworth of data and then scales it and writes a
tile file (using the same tile naming scheme as mapnik / tms.  But the tiles
I generate appear to be out about 1/4 degree too far north (look OK east
west) over the whole area.  (the mapnik generated tiles, using background
from hill shade + colour-relief and data from openstreetmap in a postgis
database are all fine and match exactly the position of tiles from the
openstreetmap server.

I am linearly scaling the raster info using the geo info from the giant tiff
and the  http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames code here
to translate between tile numbers and lon/lat.

Is linear scaling the wrong way to do this?  (but if so I would expect the
error to change as I move north - south).

I am considering using gdal2tiles to generate out the tiles, (I think I can
easily tweak the settings to generate 16bit tiffs, and then reprocess these
to fix the filenames and reformat the data ready for the android device),
but this is a rather long way around to get there.

Anyone know why this all goes awry?

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