[Gdal-dev] when are overviews displayed for a 6 inch geotiff that has gdaladdo -r average file.tif 2 4 8 16 32 64 128

[John Mitchell]

Frank,

Thanks for the information.  I am creating external overviews and would like
to use the same algorithm that you described below for internal overviews.
If I would like an overview to display if the overviews resolution is no
more than 1.2 times the requested resolution, then how can I convert from
resolution to scale so that my minimum scale within mapserver for an
overview matches the correct resolution?

Thanks,

John Mitchell

On 3/26/07, Frank Warmerdam <warmerdam at pobox.com> wrote:
>
> John Mitchell wrote:
> > Hi,
> >
> > When are overviews displayed (i.e. at what scale range will a 1/2
> > resolution overview be displayed) for a 6 inch geotiff that has gdaladdo
> > -r average file.tif 2 4 8 16 32 64 128
> > run against.
>
> John,
>
> Based on my re-reading of GDALRasterBand::OverviewRasterIO() in
> gdal/gcore/rasterio.cpp an overview level will be used if the overviews
> resolution is no more than 1.2 times the requested resolution of a
> RasterIO() request.
>
> So if you had a base layer of 30m x 30m pixels, and an overview of 60m x
> 60m
> pixels, and you made a request at 49m resolution the full resolution
> imagery
> would be used, while at 51m resolution the 60m overview would be used.
>
> Does this help?
>
> Rules may vary for format drivers that implement their own overview
> support - but this rule of thumb should apply to the generic GDAL overview
> support.
>
> Best regards,
> --
>
> ---------------------------------------+--------------------------------------
> 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 OSGeo, http://osgeo.org
>
>


-- 
John J. Mitchell
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