reprojection within map file does not seem to slow down performance within mapserver

[Ed McNierney]

John -

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!

Raster reprojection will generally be slower than vector reprojection, as
there are usually many more points involved.  But ³slower² is a relative
term.

I do raster reprojection all the time.  All TopoZone¹s imagery is stored in
UTM projections, and when a user views a topo map or aerial photo that¹s 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.

Here¹s an example:

http://www.topozone.com/map.asp?lat=40&lon=-78&datum=nad27&u=4&layer=DRG&siz
e=l&s=50

That map is centered on a UTM zone boundary.  If you scroll north or south
you¹ll 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) you¹ll 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.

     - Ed

Ed McNierney
TopoZone.com



From: John Mitchell <mitchelljj98 at GMAIL.COM>
Reply-To: John Mitchell <mitchelljj98 at GMAIL.COM>
Date: Tue, 20 Feb 2007 12:04:37 -0500
To: <MAPSERVER-USERS at LISTS.UMN.EDU>
Subject: [UMN_MAPSERVER-USERS] reprojection within map file does not seem to
slow down performance within mapserver

Hi,

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.
Has anyone else had a similar result?


 Thanks,

-- 
John J. Mitchell

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