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<TITLE>Re: [UMN_MAPSERVER-USERS] reprojection within map file does not seem to slow down performance within mapserver</TITLE>
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<FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>John -<BR>
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I agree with Frank that that documentation is somewhat misleading, but remember that it is only referring to the rendering performance of that layer. Your total MapServer turnaround time for an image request includes many different pieces, and layer rendering/reprojection is only one of them. In fact, at one point I discovered that I had tuned my data organization so well that the act of loading and parsing the map file itself was taking up 75% of my total image rendering time!<BR>
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Raster reprojection will generally be slower than vector reprojection, as there are usually many more points involved. But “slower” is a relative term.<BR>
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I do raster reprojection all the time. All TopoZone’s imagery is stored in UTM projections, and when a user views a topo map or aerial photo that’s at a UTM zone boundary, TopoZone uses MapServer to reproject one half of the image to the adjacent UTM zone on the fly. This allows us to both display imagery in UTM projection all the time, and to have no apparent gaps in coverage at UTM zone boundaries; the coverage is seamless across the US.<BR>
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Here’s an example:<BR>
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<a href="http://www.topozone.com/map.asp?lat=40&lon=-78&datum=nad27&u=4&layer=DRG&size=l&s=50">http://www.topozone.com/map.asp?lat=40&lon=-78&datum=nad27&u=4&layer=DRG&size=l&s=50</a><BR>
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That map is centered on a UTM zone boundary. If you scroll north or south you’ll be seeing on the fly reprojection of the western half of the image on every refresh. If you scroll east or west (so the zone boundary is no longer visible) you’ll be viewing maps without any reprojection involved. There are a lot of other things going on, and you should not see any perceptible difference in performance between one scrolling direction and the other.<BR>
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- Ed<BR>
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Ed McNierney<BR>
TopoZone.com<BR>
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<HR ALIGN=CENTER SIZE="3" WIDTH="95%"><B>From: </B>John Mitchell <mitchelljj98@GMAIL.COM><BR>
<B>Reply-To: </B>John Mitchell <mitchelljj98@GMAIL.COM><BR>
<B>Date: </B>Tue, 20 Feb 2007 12:04:37 -0500<BR>
<B>To: </B><MAPSERVER-USERS@LISTS.UMN.EDU><BR>
<B>Subject: </B>[UMN_MAPSERVER-USERS] reprojection within map file does not seem to slow down performance within mapserver<BR>
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Hi,<BR>
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I have noticed that reprojection within a map file does not seem to slow down performance within mapserver. The mapserver documentation states that if you have a different projection within the root of the map file and a layer then it will take 2-4 times longer to reproject, but I have not noticed any decrease in performance. <BR>
Has anyone else had a similar result?<BR>
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Thanks,<BR>
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-- <BR>
John J. Mitchell<BR>
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