[gdal-dev] Encoding EPSG:3857 (WebMercator) in GeoTIFF, and ArcGIS interoperability

[Even Rouault]

Le mercredi 15 avril 2015 22:48:45, Even Rouault a écrit :
> > The way I see it, there are two different ways to model Google Mercator:
> > 1. As a WGS84 datum/ellipsoid with a custom Mercator-based projection
> > which uses only the semi-major axis, as seems to be currently done in
> > EPSG: http://epsg-registry.org/export.htm?wkt=urn:ogc:def:crs:EPSG::3857
> > 2. As the Google spheroid with Mercator 1SP and no transformation when
> > going to WGS 84.
> > 
> > I don't think #1 is currently an option given the list of coordinate
> > transformation codes available in the GeoTIFF specification.
> 
> Right, there's no such coordinate transformation for GeoTIFF

Thinking a bit more about this issue, a dedicated coordinate transformation 
would be the cleanest way, both in GeoTIFF and proj.4 (or a flag in proj.4 to 
indicate that the spherical version of the projection method, if it exists and 
that's the case for mercator, must be used even if the ellipsoid definition is 
not spherical)
The fact that we manage currently to do WebMercator with proj.4 is due to  
WebMercator being based on WGS84 datum, and WGS84 being the "pivot" datum used 
by proj.4 when doing datum transformations. The +nadgrids=@null basically 
means that the transform from the datum defined with ellipsoid parameters 
+a=6378137 +b=6378137 and grid @null to datum WGS84 is the identity (for that 
part the values of +a and +b are completely ignored. They are only used when 
going from (long,lat) to (x,y)).
If we ever wanted to do spherical mercator on another datum than WGS84, 
there's currently no way to express that with proj.4 (except through a grid 
from the datum to WGS84 perhaps)

All that said, that doesn't really help me solving my issue, but makes me 
believe that with what currently exists in the GeoTIFF spec, there's no clean 
to express WebMercator.
- ProjectedCSTypeGeoKey = 3857 is probably the best one, although it is an 
extension of the original GeoTIFF spec, and that isn't understood properly by 
some readers
- All formulations that try to expand the definition with ProjCoordTransGeoKey 
=  CT_Mercator, its projection parameters and GCS parameters aren't really 
appropriate, since there's no way of capturing that its a spherical mercator 
that must be used.

> 
> > For #2, I would think/hope that if you specified the identity
> > 7-parameter transformation in the GeoTIFF header, ArcGIS would properly
> 
> > reproject the image:
> Hum, this will not work I'm afraid. See my response to Dmitry's to see why
> +towgs84=0,0,0,0,0,0,0 is not appropriate.
> (Plus the fact that the +towgs84 in geotiff is probably not very widely
> supported.)
> 
> > http://trac.osgeo.org/geotiff/ticket/26
> > 
> > André
> > 
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> > gdal-dev at lists.osgeo.org
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