GDAL virtual raster file

Nuno Gil nunoragil at GMAIL.COM
Thu Apr 14 16:43:02 EDT 2005

Ok, so if I understand you correctly one layer calls the original
full-res tiles and then another one call the overviews.
My overview files have an "ovr" extension and when creating the index
file for them I get

It appears no georeferencing is available for
`landsat/europe/5degvirtual/w10_n40.vrt.ovr', skipping.

On 4/14/05, Frank Warmerdam <fwarmerdam at> wrote:
> On 4/14/05, Nuno Gil <nunoragil at> 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
> >
> > 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,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at
> light and sound - activate the windows |
> and watch the world go round - Rush    | Geospatial Programmer for Rent

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