[OpenLayers-Dev] Raster map shift - solved

Stefanie Weykam stefanie at weykam.net
Sun May 9 07:18:36 EDT 2010

A couple of weeks ago I notified strange raster layer behaviour and I 
would now like to share with you how we solved the problem:

We observed a slight shift when the raster layer was displayed in 
OpenLayers. The shift seemed to be dependent on scale and GetFeatureInfo 
requests were not affected. That means that, although the pixel class 
was not the expected one, the returned value was always correct. We did 
not observe this behaviour in the thick client we use to check our map 
services and therefore concluded that the underlying WMS was correct and 
that the shift had something to do with how OpenLayers displays raster 

And the end it turned out that the test client did not display the layer 
correctly either – we just happened to look at a scale where the shift 
was very small. So, OpenLayers was off the hook and we went back to 
MapServer. We finally changed the configuration to allow non-square 
pixels - the raster layer pixels are defined in decimal degrees and thus 
are irregular.
For those who have been working for years with MapServer and didn’t even 
know that this parameter existed, this did the trick:
According to the manual, this should be the default, but it seems that 
MapServer doesn’t always ‘remember’ this.

Now everything is back in place. Thanks, Bart, for the hint.
Kind regards,

More information about the Dev mailing list