[gdal-dev] Standard Parallel 1 - being changed but not displayed?

[Even Rouault]

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

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com