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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I put the graphic together just to illustrate the differences between the extents GDAL reports for an image and what WMS uses.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There may be another explanation for the error you see. MapServer, the core any way, uses square pixels. So when an extent<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>and image dimensions are presented to the software and new extent is computed based on this assumption. This could alter<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>scale computations a touch depending on how you’re computing things. This computed extent is guaranteed to contain the requested<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>extent plus a bit more perhaps depending on the aspect ratios of the requested extent and image dimensions.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Steve<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> mapserver-dev-bounces@lists.osgeo.org [mailto:mapserver-dev-bounces@lists.osgeo.org] <b>On Behalf Of </b>Mohamed Saâd HESSANE<br><b>Sent:</b> Thursday, February 17, 2011 10:34 AM<br><b>To:</b> Stephen Woodbridge<br><b>Cc:</b> mapserver-dev@lists.osgeo.org<br><b>Subject:</b> Re: [mapserver-dev] Implementation of the msCalculateScale<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal style='margin-bottom:12.0pt'><span class=go>@Steve Lime</span> : Thank you for the graphic. So if i understand, the map is generated with GDAL, which use the big extent, and the scale and scalebar generated from mapeserver, which calculate it with the small extent.<br>According to my tests, it generate a 0.1% error in a 1/300000 scale. It's not bad. But the best would be not have this error.<br><br><o:p></o:p></p><div><p class=MsoNormal>2011/2/17 Stephen Woodbridge <<a href="mailto:woodbri@swoodbridge.com">woodbri@swoodbridge.com</a>><o:p></o:p></p><p class=MsoNormal>I thought there were some extensive discussions on msCalculateScale and this particular issue in the past. I don't remember if it was on the dev-list or in a ticket. If I recall, it was changed which caused significant regressions and the changed back because of them.<br><br>I'm not opposed to a change, per say, but I would like to avoid the issues we had last time. If this is getting into 6.0, this would be a good departure point to make a backwards compatibility breaking change. But once 6.0 is released, we will need to maintain some compatibility within the 6.x family of releases and not introduce major compatibility issues.<br><br>-Steve W<o:p></o:p></p><div><p class=MsoNormal><br><br>On 2/17/2011 10:17 AM, Jan Hartmann wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><p class=MsoNormal>Interesting. Could this be added to the documentation? It's important<br>when you are scaling at pixel level. Would it make sense to add an<br>option for MapServer to treat extents the same way as GDAL?<br><br>Jan<br><br>On 2011-02-17 16:07, Lime, Steve D (DNR) wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in'><div><p class=MsoNormal><br>It’s due to how MapServer interprets extents. It uses a pixel-center<br>model so you end up using width or height minus one in computations<br>because<br><br>an extent is from the center of the UL pixel to the center of LR pixel<br>(you lose the “one” from the outside) . I’ve attached a graphic I<br>created some time<br><br>ago that illustrates the difference between a MapServer extent and the<br>model GDAL and WMS use.<br><br>Steve<br><br>*From:*<a href="mailto:mapserver-dev-bounces@lists.osgeo.org" target="_blank">mapserver-dev-bounces@lists.osgeo.org</a><br><mailto:<a href="mailto:mapserver-dev-bounces@lists.osgeo.org" target="_blank">mapserver-dev-bounces@lists.osgeo.org</a>><br>[mailto:<a href="mailto:mapserver-dev-bounces@lists.osgeo.org" target="_blank">mapserver-dev-bounces@lists.osgeo.org</a>] *On Behalf Of *Mohamed<br>Saâd HESSANE<br>*Sent:* Thursday, February 17, 2011 4:41 AM<o:p></o:p></p></div><p class=MsoNormal>*To:* <a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a> <mailto:<a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a>><o:p></o:p></p><div><p class=MsoNormal><br>*Subject:* [mapserver-dev] Implementation of the msCalculateScale<br><br>Good morning everybody.<br>I don't understand how the msCalculateScale function in mapscale.c is<br>implemented.<br>As far as I know, the scale is calculated from the relation :<br>inches_in_screen / inches_in_reallity.<br>inches_in_screen : depend on the DPI and the width of the map;<br>inches_in_reality : depend on the map.extent, and the pixel size.<br>So why the "width-1" at the line 96? For me just "width" is more<br>correct. Perhaps I'm wrong.<br>The branches 4.X have just "width" in their implementations (line 125).<br>So why the center pixel is removed from calculation?<br><br><br>_______________________________________________<br>mapserver-dev mailing list<o:p></o:p></p></div><p class=MsoNormal><a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a> <mailto:<a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a>><o:p></o:p></p><div><p class=MsoNormal><br><a href="http://lists.osgeo.org/mailman/listinfo/mapserver-dev" target="_blank">http://lists.osgeo.org/mailman/listinfo/mapserver-dev</a><o:p></o:p></p></div></blockquote><div><p class=MsoNormal><br><br>_______________________________________________<br>mapserver-dev mailing list<br><a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a><br><a href="http://lists.osgeo.org/mailman/listinfo/mapserver-dev" target="_blank">http://lists.osgeo.org/mailman/listinfo/mapserver-dev</a><o:p></o:p></p></div></blockquote><div><div><p class=MsoNormal><br>_______________________________________________<br>mapserver-dev mailing list<br><a href="mailto:mapserver-dev@lists.osgeo.org" target="_blank">mapserver-dev@lists.osgeo.org</a><br><a href="http://lists.osgeo.org/mailman/listinfo/mapserver-dev" target="_blank">http://lists.osgeo.org/mailman/listinfo/mapserver-dev</a><o:p></o:p></p></div></div></div><p class=MsoNormal><o:p> </o:p></p></div></body></html>