[mapserver-dev] Questions regarding to the extent/scale calculations in MapServer

[Lime, Steve D (MNIT)]

The model was based on how ERDAS represented pixels back when MapServer was first written. I was a satellite image processing guy at the time and that was the initial focus of the software. Early code used the ERDAS C toolkit which reinforced the model. Personally I think the center of a pixel model makes more sense.

Changing would probably be *very* disruptive. Not so much within MapServer code since the areas of change are pretty isolated in a few macros and transformations in the OWS code. The change would affect every mapfile that sets or uses scale denominators. Plus, clients would need to be updated and need to be made version aware.

Steve

From: mapserver-dev-bounces at lists.osgeo.org [mailto:mapserver-dev-bounces at lists.osgeo.org] On Behalf Of Tamas Szekeres
Sent: Wednesday, November 27, 2013 8:54 AM
To: mapserver-dev at lists.osgeo.org
Subject: [mapserver-dev] Questions regarding to the extent/scale calculations in MapServer

Hi All,

We've already noticed MapServer use a "center of pixel" representation when doing the extent/scale calculations in the code, which may cause quite some confusion for the users (mostly from the mapscript side) regarding to the behaviour.

The most typical issue I've encountered is the complain about "why mapserver modifies my accurate extent specified in setExtent and why MapServer calculates a different scale I can calculate?"

The reason of why is in fact that we consider the area coverage of the image is larger than the area coverage of the map extent (half of the pixel size in each directions). But the users (and mostly everyone in the world except MapServer) considers that the area coverage of the image is the same as the area coverage of the map extent.

Can someone explain why we do things this way and do we have the chance to get rid of it?

We could also eliminate the unnecessary transformations done in the WMS/WCS interface where the extent of the BBOX is considered to be in - let's say - edge of pixel representation and not in center of pixel representation.

Best regards,

Tamas


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