Performance hints please

[Stephen Davies]

Thanks Frank. Now I understand.
As luck would have it, the maps which are biggest (highest resolution 
rasters) are also the ones with the smallest extents so they just miss 
going over the step from base map to 50% overview. 
These base maps are 1m/pixel and the final maps are just under 2.
I shall try experimenting with the extents (currently determined from 
property boundaries) to see if I can push them over.

However, several other maps are seeing a significant improvement.

Cheers and thanks again,
Stephen

On Thursday 20 April 2006 13:35, Frank Warmerdam wrote:
> 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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