Hi Even,
Thanks for the reply.
I guess GDAL chose Mercator_1SP because that's the one that's explicitly
defined in the projection text.
I've had a quick look at the two files using listgeo, and highlighted the
different lines with **** prefix below (it seems GDAL removes 6 tags, and adds
one).
The before:
Keyed_Information:
GTModelTypeGeoKey (Short,1): ModelTypeProjected
GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
GTCitationGeoKey (Ascii,9): "Mercator"
GeographicTypeGeoKey (Short,1): GCS_WGS_84
GeogCitationGeoKey (Ascii,7): "WGS 84"
****GeogGeodeticDatumGeoKey (Short,1): Datum_WGS84
****GeogLinearUnitsGeoKey (Short,1): Linear_Meter
GeogAngularUnitsGeoKey (Short,1): Angular_Degree
****GeogEllipsoidGeoKey (Short,1): Ellipse_WGS_84
GeogSemiMajorAxisGeoKey (Double,1): 6378137
****GeogSemiMinorAxisGeoKey (Double,1): 6356752.31424518
GeogInvFlatteningGeoKey (Double,1): 298.257223563
ProjectedCSTypeGeoKey (Short,1): User-Defined
****PCSCitationGeoKey (Ascii,9): "Mercator"
ProjectionGeoKey (Short,1): User-Defined
ProjCoordTransGeoKey (Short,1): CT_Mercator
ProjLinearUnitsGeoKey (Short,1): Linear_Meter
****ProjStdParallel1GeoKey (Double,1): 60
ProjNatOriginLongGeoKey (Double,1): 0
ProjNatOriginLatGeoKey (Double,1): 0
ProjFalseEastingGeoKey (Double,1): 0
ProjFalseNorthingGeoKey (Double,1): 0
End_Of_Keys.
------------
The after:
Keyed_Information:
GTModelTypeGeoKey (Short,1): ModelTypeProjected
GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
GTCitationGeoKey (Ascii,9): "Mercator"
GeographicTypeGeoKey (Short,1): GCS_WGS_84
GeogCitationGeoKey (Ascii,7): "WGS 84"
GeogAngularUnitsGeoKey (Short,1): Angular_Degree
GeogSemiMajorAxisGeoKey (Double,1): 6378137
GeogInvFlatteningGeoKey (Double,1): 298.257223563
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
End_Of_Keys.
End_Of_Geotiff.
I'm not clear on why the things that changed were changed, but I can see that
GDAL removed the ProjStdParallel1GeoKey and value. Should not that have been
displayed by GDALSRSInfo for the Before file, even if it was wrongly set for
this given projection?
Thanks,
Jonathan
---- On Tue, 17 Nov 2015 09:18:49 -0800 Even
Rouault<even.roua...@spatialys.com> wrote ----
Le mardi 17 novembre 2015 17:55:15, Jonathan Moules a écrit :
> Hi List,
> I have a Geotiff which includes this projection:
>
> PROJ.4 : '+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m
> +no_defs '
>
>
> OGC WKT :
> PROJCS["Mercator",
> GEOGCS["WGS 84",
> DATUM["WGS_1984",
> SPHEROID["WGS 84",6378137,298.257223563,
> AUTHORITY["EPSG","7030"]],
> AUTHORITY["EPSG","6326"]],
> PRIMEM["Greenwich",0],
> UNIT["degree",0.0174532925199433],
> AUTHORITY["EPSG","4326"]],
> 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"]]]
>
>
>
>
> If I load this raster into ArcGIS, it displays in the wrong place (a few
> thousand kilometres North).
>
>
> I then run it through gdal_translate (GDAL 1.11.1), with no flags:
> * gdal_trainslate input.tif output.tif
>
> For output.tif, GDALSRSInfo shows that the projection is identical, but
now
> the file loads correctly in ArcGIS. The same file works fine in QGIS both
> before and after the "translation".
>
>
> Looking at the projection info in ArcGIS, it displays one difference:
> Before (not working):
> Standard_parallel_1 = 60
>
>
> After (working):
> Standard_parallel_1 = 0
>
>
> But I don't see anything about those in either of the GDALSRSInfo outputs.
>
>
> So my questions:
> - What is gdal_translate doing to the file to "fix" it?
> - If it is something to do with Standard Parallel 1 - why isn't this
> component of the projection exposed by GDAL?
Yes, in Mercator_1SP, there's no Standard Parallel 1, this is for
Mercator_2SP.
See
http://www.remotesensing.org/geotiff/proj_list/mercator_1sp.html
http://www.remotesensing.org/geotiff/proj_list/mercator_2sp.html
I guess your original geotiff file has some unusual formulation which is
detected as Mercator_1SP by GDAL, and probably Mercator_2SP by ArCGIS.
You could try with the listgeo utility that comes with libgeotiff to display
the geotiff keys.
>
> Thoughts welcome.
> Thanks,
> Jonathan
--
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http://www.spatialys.com
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