On 2015-04-15 14:59, Even Rouault wrote:
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
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.
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:
http://trac.osgeo.org/geotiff/ticket/26
André
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