GDAL virtual raster file

[Frank Warmerdam]

On 4/14/05, Nuno Gil <nunoragil at gmail.com> wrote:
> Dear Frank/Listers,
> There was no problem in creating the virtual files or the shape index file.
> The problem now is with the overviews... Each original Landsat file is
> about 300 Mb, so when MapServer is requesting each tile it is asking
> for a 300 x 3 Mb file (RGB bands).
> I created overviews with "gdaladdo -r average cover.vrt 5 10 50 100"
> but when trying to access all of them (100 landsat images) in the
> webclient it takes a lot of time. If I first zoom in (lets say the UK)
> and then request the layer (Europe Landsat 5) the performance is ok.
> You can take a look at
> http://iceds.ge.ucl.ac.uk:8080/viewer/iceds/index.html
> Could this have to do with the overviews? Perhaps not enough levels?
> Can it be also the number of tiles MapServer is requesting at the same
> time (100 tiles)? Should I aggregate them first?

Nuno, 

The traditional approach to this is to prepare composite "overviews"
that would apply at large scales.  So you would set a MAXSCALE
on the tile indexed layer at some reasonable level (perhaps a scale
were the user is likely to be seeing more than 9 images or so.  And
then create a new layer - the overview version  - with it's MINSCALE
set to the same thing.   This layer can be just a single file at a very
much reduced resolution - matching the scale at which it kicks in.

I would suggest actually using MapServer to generate it from the
other layer -- perhaps using shp2img or just doing it on the web.

Now, I am fluffing over lots of details of how to compute the scales,
and build the alternative overview but then I get confused about
those details. :-)  They are solvable but a bit messy.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent