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 <[EMAIL PROTECTED]> 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, [EMAIL PROTECTED] 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
