[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

[Frank Warmerdam]

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,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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