[Mapserver-dev] Fwd: Problem cascading WMS servers that support non-square pixel requests

[Steve Lime]

>>> Barry Wilson <barrywilson at fs.fed.us> 6/17/2004 3:49:30 PM >>>

Through an extended e-mail exchange with Ed McNierney, I think I've
pinpointed a problem with the way Mapserver cascades fully-compliant
WMS
servers, i.e those that support non-square pixel requests.  See the
thread
titled "OGC compliance for WMS" dated 6/8/04.  In that thread I
described
the problem this way:

"In the example I described, Mapserver is acting both as client and
server.
It is a client to the USGS WMS and a server of that data along with
several
local data layers.  The local layers are not reprojected because their
native projection is EPSG:2163.  The NED layer from the USGS WMS is
indeed
reprojected from EPSG:4326 to EPSG:2163.  However, it appears that the
image generated by the USGS WMS was stretched, *prior* to reprojection
by
Mapserver, as per the WMS specification because the BBOX aspect ratio
and
the output width/height differ.  The reason they differ is because the
BBOX
passed to Mapserver was based on EPSG:2163, while the BBOX that
Mapserver
passes to the USGS WMS is based on EPSG:4326. The issue is not that
Mapserver is "filling in" to preserve the correct aspect ratio, it is
that
the USGS WMS doesn't and instead stretches, as per the spec.  Perhaps
it's
simply a matter of having Mapserver, when cascading other WMSes, adjust
the
BBOX that it passes to those servers to essentially force *them* to
fill in
to maintain the correct aspect ratio so that no stretching takes
place."

I believe Frank Warmerdam opened a related bug report (#728) in
response to
this thread.  However, I don't know if Mapserver support of non-square
pixel requests will address this particular issue.  In the case I've
described, the request made to Mapserver would produce square pixels
for
local data and non-square pixels for the data cascaded from USGS since
it
involves a reprojection that changes the aspect ratio of the bounding
box.

-Ty

~~~~~~~~~~~~~~~~~~~~~~~~
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 
~~~~~~~~~~~~~~~~~~~~~~~~