Frank,<br><br>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?<br><br>Thanks,<br><br>John Mitchell<br><br><div>
<span class="gmail_quote">On 3/26/07, <b class="gmail_sendername">Frank Warmerdam</b> <<a href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
John Mitchell wrote:<br>> Hi,<br>><br>> When are overviews displayed (i.e. at what scale range will a 1/2<br>> resolution overview be displayed) for a 6 inch geotiff that has gdaladdo<br>> -r average file.tif
2 4 8 16 32 64 128<br>> run against.<br><br>John,<br><br>Based on my re-reading of GDALRasterBand::OverviewRasterIO() in<br>gdal/gcore/rasterio.cpp an overview level will be used if the overviews<br>resolution is no more than
1.2 times the requested resolution of a<br>RasterIO() request.<br><br>So if you had a base layer of 30m x 30m pixels, and an overview of 60m x 60m<br>pixels, and you made a request at 49m resolution the full resolution imagery
<br>would be used, while at 51m resolution the 60m overview would be used.<br><br>Does this help?<br><br>Rules may vary for format drivers that implement their own overview<br>support - but this rule of thumb should apply to the generic GDAL overview
<br>support.<br><br>Best regards,<br>--<br>---------------------------------------+--------------------------------------<br>I set the clouds in motion - turn up | Frank Warmerdam, <a href="mailto:warmerdam@pobox.com">warmerdam@pobox.com
</a><br>light and sound - activate the windows | <a href="http://pobox.com/~warmerdam">http://pobox.com/~warmerdam</a><br>and watch the world go round - Rush | President OSGeo, <a href="http://osgeo.org">http://osgeo.org
</a><br><br></blockquote></div><br><br clear="all"><br>-- <br>John J. Mitchell