[mapserver-users] WMS Client changes slightly BBOX

[Lime, Steve D (MNIT)]

There's a difference between the extent models used by MapServer and OGC. The WMS code adjusts for that difference so the input extent will definitely see adjustment to account for that. OGC uses the UL corner of the UL pixel and the LR corner of the LR pixel. MapServer uses pixel centers consistently. Do your scale computations account for this difference?

Steve

From: mapserver-users-bounces at lists.osgeo.org [mailto:mapserver-users-bounces at lists.osgeo.org] On Behalf Of Peter Hopfgartner
Sent: Thursday, July 24, 2014 7:20 AM
To: mapserver-users at lists.osgeo.org
Subject: Re: [mapserver-users] WMS Client changes slightly BBOX

Hi Worth,

thanks for your quick reply. Anyway, the ratio of the extent and of the image seem to match al least up to 10 digits. Playing with the numbers, the error is in the same order of magnitude as if I change image size by one pixel. I would guess, that there is a "off by 1" error somewhere in the MapServer code.

Regards,

Peter

On 07/24/2014 01:30 PM, Worth Lutz wrote:
Peter,

I have found that if the map size (width and height in pixels) aspect ratio does not exactly match the map extents (map units) aspect ratio the map extents are adjusted.

I'm working with MapScript and to get exact scale pdf map I have to set map size first and then extent and then scale to get good results.

I hope this helps.


Worth Lutz
------------------
On 07/24/2014 06:49 AM, Peter Hopfgartner wrote:
I'm generating dynamically Map Object, with with a certain extent, in the concrete example
629396.05065523,4984079.9894638,655192.92565523,5019084.3644638

In generated Map object uses the WMS client functionality to call WMS services. Anyway, the BBOX get's slightly changed, so that the original scale of 1:100000 gets changed to approx 1:100103, since it calls the WMS services with
629382.807906257,4984062.02007212,655206.168404203,5019102.33385548
as can be taken from the web server log files:

mmm.mmm.mmm.mmm - - [24/Jul/2014:12:25:58 +0200] "GET /author/services/ows.php?PROJECT=ancitel&MAP=catasto_reti&LANG=xw&OGSTAT=ot3cp0i7qmph4guuof75702m70&RESOLUTION=96&LAYERS=dtm,uso_lombardia,strade_lombardia,g_limiti_amministrativi,g_label,g_0707,g_0704&REQUEST=GetMap&SERVICE=WMS&FORMAT=image/png%3B+mode%3D8bit&STYLES=,,,,,,&HEIGHT=1323&VERSION=1.1.1&SRS=EPSG:32632&WIDTH=975&BBOX=629382.807906257,4984062.02007212,655206.168404203,5019102.33385548&TRANSPARENT=TRUE&EXCEPTIONS=application/vnd.ogc.se_inimage HTTP/1.1" 200 678803 "-" "MapServer/6.4.1 libcurl/7.19.7"

This change in scale is beyound te axpected tolerance in this particular use case, where technical data must be plotted in a PDF file.

Any help is appreciated. Obvioulsly, I can fetch the WMS services in my code and overlay them "by hand". Anyway, for a couple of reasons this was the preferred approach.

Regards,

Peter





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