[gdal-dev] Fwd: GeoPKG Zoom Level Calculation

[Roarke Gaskill]

Even,

Thanks for your reply.  I get it now.  :)

I am new to gdal in general.  For context, why does CreateCopy() choose the
nearest zoom level and not SetGeoTransform()?  As a new user, it seems to
me the ZOOM_LEVEL_STRATEGY should apply to both the SetGeoTransform() and
CreateCopy().

Thanks again,
Roarke

On Wed, Sep 13, 2017 at 3:50 PM, Even Rouault <even.rouault at spatialys.com>
wrote:

> On mercredi 13 septembre 2017 15:38:55 CEST Roarke Gaskill wrote:
>
> > Hi,
>
> >
>
> > In the GDALGeoPackageDataset
>
> > <https://github.com/OSGeo/gdal/blob/65dea1c999b6b5f692c945ea074b04
> cb5e43eca4
>
> > /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/65dea1c999b6b5f692c945ea074b04
> cb5e43eca4
>
> > /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
>
>
>
> --
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com
>



-- 
*Roarke **Gaskill  *|Senior Software Engineer
*e:* roarke.gaskill at weather.com
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