[gdal-dev] Fwd: GeoPKG Zoom Level Calculation

[Even Rouault]

On mercredi 13 septembre 2017 15:38:55 CEST Roarke Gaskill wrote:
> Hi,
> 
> In the GDALGeoPackageDataset
> <https://github.com/OSGeo/gdal/blob/65dea1c999b6b5f692c945ea074b04cb5e43eca4
> /gdal/ogr/ogrsf_frmts/gpkg/ogrgeopackagedatasource.cpp#L1819> class it
> appears the zoom level in the SetGeoTransform function does not calculate
> the zoom level correctly.  Also, it is different from the zoom level
> calculation in CreateCopy
> <https://github.com/OSGeo/gdal/blob/65dea1c999b6b5f692c945ea074b04cb5e43eca4
> /gdal/ogr/ogrsf_frmts/gpkg/ogrgeopackagedatasource.cpp#L4079> function.  The
> zoom level calculation in the CreateCopy appears to work as I would expect.
> 
> The transform I am using to test is:
> [-20037508.342789248, 6176.637812966647, 0.0, 19971868.880408563, 0.0,
> -6177.590116052211]
> 
> The CreateCopy zoom level logic produces level 5.  Which is what I would
> expect.
> 
> The SetGeoTransform zoom level logic is not able to identify a zoom level.
> 
> Is the SetGeoTransform zoom level calculation logic wrong?

Roarke,

when using SetGeoTransform() and a pre-defined tiling scheme (here I assume you must use 
GoogleMapsCompatible), the resolution of the geotransform matrix passed to 
SetGeoTransform() must exactly match the base resolution of the tiling scheme divided by a 
power of 2

>>> 156543.0339280410 / 6176.637812966647
25.344376450147266

which is not a power of 2.

CreateCopy() in the GPKG implementation with a pre-defined tiling scheme identifies the 
closest zoom level for your source dataset and then resample it  so its resolution matches 
exactly one of the zoom levels.

Even

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