[Mapserver-users] OGC compliance for WMS

[Barry Wilson]

I've been reading through the archives and saw a couple of threads related
to Mapserver OGC compliance for WMS dated from November 03 and January 04.
Has the issue of "non-square" pixels been or is it being addressed?

This can be a fairly significant problem for folks who want to be able to
cascade from fully compliant WMSes.  A concrete example is the set of
services provided by USGS, say something like the NED.  These data are
served up by ArcIMS in EPSG:4326, a geographic SRS.  When they are cascaded
through my UMN Mapserver WMS, I reproject them to EPSG:2163, a Lambert
Azimuthal Equal Area projection.  This is where the "non-square" pixels
issue shows up.  When a client requests data from my WMS using a bounding
box in EPSG:2163, the bounding box that gets passed along to the USGS WMS
will have a different aspect ratio in EPSG:4326.  However, the output image
aspect ratio remains fixed.  Therefore, the image from the USGS WMS gets
stretched.  Any data that I serve up locally and overlay on the USGS image
will not get stretched.  The end result is that the data from the two
sources will line up only in the middle of the image.  The conflation gets
worse the further you get from the center.

The simple solution, of course, is that I serve up my data in EPSG:4326 as
well.  That will work in this instance, but I can see problems arising if I
also want to pull in data from other WMSes that serve up data in some other
SRS.

-Ty

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B. Tyler Wilson
USDA Forest Service
North Central Research Station
1992 Folwell Avenue
St. Paul, MN  55108
651-649-5189  barrywilson at fs.fed.us
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