[gdal-dev] [PATCH v2] Support Mercator_2SP in GeoTIFF

[Even Rouault]

Selon Trent Piepho <tpiepho at gmail.com>:

Thanks. Would you mind attaching it in a ticket in GDAL trac :
http://trac.osgeo.org/gdal/newticket ?
I see you've created the ticket for the corresponding libgeotiff part. Before
applying the GDAL part, I think we should wait for Frank having reviewed and
applied the libgeotiff part, so that we can have round-tripping.


> Mercator_[12]SP are both the same projection in GeoTIFF.  Lacking a
> definition in the official specification, the intention appears to be that
> for Mercator_2SP the latitude of true scale (lat_ts) should be specified in
> ProjStdParallel1GeoKey and for Mercator_1SP the scale at origin (k) should
> be specified in ProjScaleAtNatOriginGeoKey.  As proposed here,
> http://permalink.gmane.org/gmane.comp.gis.geotiff/387
>
> Current behavior when creating a GTiff with 2SP is to drop lat_ts and create
> a default k value of 1.0, which will product an incorrect projection.  When
> reading, 1SP is always used and lat_ts is ignored.  It is expected that
> libgeotiff will provide a default k of 1.0 if none was specified in the
> file.
>
> This patches changes the behavior when writing to supply
> ProjScaleAtNatOriginGeoKey for Mercator_1SP or ProjStdParallel1GeoKey for
> Mercator_2SP.
>
> When reading, 2SP is used if ProjStdParallel1GeoKey is present and 1SP is
> used otherwise.  A warning is issued if both keys are present (and scale is
> ignored).  It is expected that libgeotiff's normalization will continue to
> provide a default scale of 1.0 if neither parameter is present.
> ---
> v2
> Split out libgeotiff part.  This patch does not need the libgeotiff
> patch to be applied.  Will write correctly without libgeotiff patch.
> Reading will be unchanged unless libgeotiff is also patched.
>
>  gdal/frmts/gtiff/gt_wkt_srs.cpp |   28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/gdal/frmts/gtiff/gt_wkt_srs.cpp
> b/gdal/frmts/gtiff/gt_wkt_srs.cpp
> index 5934df5..0afb101 100644
> --- a/gdal/frmts/gtiff/gt_wkt_srs.cpp
> +++ b/gdal/frmts/gtiff/gt_wkt_srs.cpp
> @@ -737,9 +737,21 @@ char *GTIFGetOGISDefn( GTIF *hGTIF, GTIFDefn * psDefn )
>              break;
>
>            case CT_Mercator:
> -            oSRS.SetMercator( adfParm[0], adfParm[1],
> -                              adfParm[4],
> -                              adfParm[5], adfParm[6] );
> +            /* If a lat_ts was specified use 2SP, otherwise use 1SP */
> +            if (psDefn->ProjParmId[2] == ProjStdParallel1GeoKey)
> +            {
> +                if (psDefn->ProjParmId[4] == ProjScaleAtNatOriginGeoKey)
> +                    CPLError( CE_Warning, CPLE_AppDefined,
> +                              "Mercator projection should not define both
> StdParallel1 and ScaleAtNatOrigin.\n"
> +                              "Using StdParallel1 and ignoring
> ScaleAtNatOrigin.\n" );
> +                oSRS.SetMercator2SP( adfParm[2],
> +                                     adfParm[0], adfParm[1],
> +                                     adfParm[5], adfParm[6]);
> +            }
> +            else
> +                oSRS.SetMercator( adfParm[0], adfParm[1],
> +                                  adfParm[4],
> +                                  adfParm[5], adfParm[6] );
>
>              if (psDefn->Projection == 1024 || psDefn->Projection == 9841) //
> override hack for google mercator.
>              {
> @@ -1510,14 +1522,18 @@ int GTIFSetFromOGISDefn( GTIF * psGTIF, const char
> *pszOGCWKT )
>          GTIFKeySet(psGTIF, ProjNatOriginLongGeoKey, TYPE_DOUBLE, 1,
>                     poSRS->GetNormProjParm( SRS_PP_CENTRAL_MERIDIAN, 0.0 ) );
>
> -        GTIFKeySet(psGTIF, ProjScaleAtNatOriginGeoKey, TYPE_DOUBLE, 1,
> -                   poSRS->GetNormProjParm( SRS_PP_SCALE_FACTOR, 1.0 ) );
> -
>          GTIFKeySet(psGTIF, ProjFalseEastingGeoKey, TYPE_DOUBLE, 1,
>                     poSRS->GetProjParm( SRS_PP_FALSE_EASTING, 0.0 ) );
>
>          GTIFKeySet(psGTIF, ProjFalseNorthingGeoKey, TYPE_DOUBLE, 1,
>                     poSRS->GetProjParm( SRS_PP_FALSE_NORTHING, 0.0 ) );
> +
> +        if( EQUAL(pszProjection,SRS_PT_MERCATOR_2SP) )
> +            GTIFKeySet(psGTIF, ProjStdParallel1GeoKey, TYPE_DOUBLE, 1,
> +                       poSRS->GetNormProjParm( SRS_PP_STANDARD_PARALLEL_1,
> 0.0 ) );
> +        else
> +            GTIFKeySet(psGTIF, ProjScaleAtNatOriginGeoKey, TYPE_DOUBLE, 1,
> +                       poSRS->GetNormProjParm( SRS_PP_SCALE_FACTOR, 1.0 ) );
>      }
>
>      else if( EQUAL(pszProjection,SRS_PT_OBLIQUE_STEREOGRAPHIC) )
> --
> 1.7.10.4
>
>