RFC 25: extents model

[Frank Warmerdam]

Steve,

I have reviewed RFC 25:

   http://mapserver.gis.umn.edu/development/rfc/ms-rfc-25/

This primarily addresses changing the map extents to go from the top left
of the top left pixel to the bottom right of the bottom right pixel rather
than the current model of going from the center of the top left pixel to
the center of the bottom right pixel.

While I am, by god given and genetically pre-ordained orientation, a
"pixel as area" man and so feel this would be a step towards upright living
and moral rectitude, I am concerned that the provided rationale is not
strong, and that it is likely to cause disruption and pain.

First, I would call into question the assumption that the OGC specifications
are predominantly based on "pixel as area" assumptions.  Apparently this issue
was vague in the WCS 1.0 specification, but in WCS 1.1 the position has clearly
been taken that extents (expressed in coverage descriptions, and I would assume
the BBOX equivelent) are from center of pixel to center of pixel.  So, at least
one significant OGC specification is better aligned with the existing
implementation.

Second, there is a lot of code doing pixel/line to georef and back
transformations in the mapdrawgdal.c, and related raster rendering files.
Over the last several years I have had to twiddle with it a number of
times to ensure that the existing mapserver extent model is properly
accounted for.  And I'm very hesitant to change it again.  I have a lack
of confidence in my ability to change all the right places in the code
raster rendering code.

I would prefer to see RFC 25 re-affirm the current model, documenting
the assumptions for extents and the various map.h macros clearly.  And
also have us do some sort of review to ensure that WCS and other services
properly take this into account.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org