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

[Even Rouault]

Le mercredi 15 avril 2015 18:21:51, xavier lhomme a écrit :
> Dis you try with 102113 or 102100  instead of 3857 ? I had once to use
> those code in order to be discovered as 3857 in arcgis. when i created
> geotiff with gdal warp/transform.

Xavier,

thanks for the hint. I've just tried it and thinks it qualifies as yet another 
not completely satisfactory solution.

Using -a_srs EPSG:102113 effectively results in a correctly placed geotiff in 
ArcGIS (with a warning about it being in GCS_WGS_1984_Major_Auxiliary_Sphere), 
but there are several points that are not that great :
- this is not a standard EPSG code
- due to being > 32767, it is not encoded in ProjectedCSTypeGeoKey, which was 
one of my "requirement"
- when read by GDAL, the expanded GeoTIFF definition isn't properly identified 
as WebMercator, consequently the proj.4 string lacks the "+nadgrids=@null" 
stuff, which makes it a non-complete datum definition and cause issues when 
reprojecting such GeoTIFF with GDAL (lack of datum transformation from/to the 
WGS84 datum of WebMercator). That could admitedly being tweaked in GDAL.

For the record, the GeoTIFF keys written by GDAL are:
{{{
      GTModelTypeGeoKey (Short,1): ModelTypeProjected
      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
      GTCitationGeoKey (Ascii,22): "WGS_1984_Web_Mercator"
      GeographicTypeGeoKey (Short,1): User-Defined
      GeogCitationGeoKey (Ascii,151): "GCS Name = 
GCS_WGS_1984_Major_Auxiliary_Sphere|Datum = WGS_1984_Major_Auxiliary_Sphere|
Ellipsoid = WGS_1984_Major_Auxiliary_Sphere|Primem = Greenwich|"
      GeogGeodeticDatumGeoKey (Short,1): User-Defined
      GeogAngularUnitsGeoKey (Short,1): Angular_Degree
      GeogEllipsoidGeoKey (Short,1): User-Defined
      GeogSemiMajorAxisGeoKey (Double,1): 6378137          
      GeogSemiMinorAxisGeoKey (Double,1): 6378137          
      GeogPrimeMeridianLongGeoKey (Double,1): 0                
      ProjectedCSTypeGeoKey (Short,1): User-Defined
      ProjectionGeoKey (Short,1): User-Defined
      ProjCoordTransGeoKey (Short,1): CT_Mercator
      ProjLinearUnitsGeoKey (Short,1): Linear_Meter
      ProjNatOriginLongGeoKey (Double,1): 0                
      ProjNatOriginLatGeoKey (Double,1): 0                
      ProjFalseEastingGeoKey (Double,1): 0                
      ProjFalseNorthingGeoKey (Double,1): 0                
      ProjScaleAtNatOriginGeoKey (Double,1): 1  
}}}

And GDAL reads that as :
{{{
PROJCS["WGS_1984_Web_Mercator",
    GEOGCS["GCS_WGS_1984_Major_Auxiliary_Sphere",
        DATUM["WGS_1984_Major_Auxiliary_Sphere",
            SPHEROID["WGS_1984_Major_Auxiliary_Sphere",6378137,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
}}}
(not sure why it lacks the latitude_of_origin,  but that's another minor 
issue)

ArcCatalog identifies the SRS as:
{{{
WGS_1984_Web_Mercator
WKID: 3785 Authority: EPSG

Projection: Mercator
false_easting: 0.0
false_northing: 0.0
central_meridian: 0.0
standard_parallel_1: 0.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_WGS_1984_Major_Auxiliary_Sphere
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984_Major_Auxiliary_Sphere
  Spheroid: WGS_1984_Major_Auxiliary_Sphere
    Semimajor Axis: 6378137.0
    Semiminor Axis: 6378137.0
    Inverse Flattening: 0.0
}}}

Even

> 
> Le 15 avr. 2015 18:16, "Even Rouault" <even.rouault at spatialys.com> a écrit :
> > Hi,
> > 
> > I've collected in http://trac.osgeo.org/gdal/ticket/5924 a summary of
> > issues
> > and findings when trying to write GeoTIFF files in EPSG:3857 in a
> > "standard" way
> > (using ProjectedCSTypeGeoKey = 3857), while making them correctly
> > displayed by
> > ArcGIS (and ideally by other independant implementations).
> > The current situation is that there's no such way (AFAIK). I'd appreciate
> > any
> > feedback/suggestion related to that issue.
> > 
> > Best regards,
> > 
> > Even
> > 
> > --
> > Spatialys - Geospatial professional services
> > http://www.spatialys.com
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> > gdal-dev mailing list
> > gdal-dev at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/gdal-dev

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