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

Tamas Szekeres szekerest at gmail.com
Wed Nov 27 06:53:53 PST 2013

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

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,

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