Performance hints please

[Frank Warmerdam]

Stephen Davies wrote:
> Yes, I see the overviews in the gdalinfo output.
> So far, I have only used 50%. Any guides as to what is a good number of 
> overviews to create?
> How does mapserv chose which overview to use? What is "appropriate"?


Stephen,

MapServer (well actually GDAL does this without mapserver even realizing
it) will use an overview if it is at least as fine a resolution as the
map request.  So, if the pixel size of your base file is 2m x 2m and you
select a "2" overview with gdaladdo it will have a pixel size of 4m x 4m.

If you requested a map with a pixel size of 3m x 3m the mapserver will use
the base image (2m x 2m) and decimate from that.  But if you ask for a map
with 4.5m x 4.5m pixels then the overview would get used.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGF, http://osgeo.org