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

[Hermann Peifer]

On 30/10/2011 20:16, Iain wrote:
> 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.


- OSM tiles come in a *Spherical* Mercator projection
- what you observe seems to be a latitude-only shift

This type of "datum" shift often occurs when projecting from a sphere to 
an ellpsoid or vice-versa, although 1/4 degree (~28 km) sounds like 
quite a lot.

Here 2 links to recent gdal-dev postings on this issue:

http://osgeo-org.1803224.n2.nabble.com/gdal-dev-Ogr2ogr-projection-problem-features-are-shifted-12-miles-to-the-north-How-do-I-fix-it-td6779672.html

http://osgeo-org.1803224.n2.nabble.com/gdal-dev-Ogr2ogr-datum-shift-td6795580.html