[Qgis-user] Re: Inconsistent projection parameters for EPSG:3168

[Hilmy Hashim]

The wrong value for the false_easting for the RSO projection has been
confirmed as a bug in GeoTiff: http://trac.osgeo.org/geotiff/ticket/38

*Hilmy*


On Thu, Apr 7, 2011 at 5:04 PM, Hilmy Hashim <hilmyh at gmail.com> wrote:

> I am using QGIS 1.7.0 dev version (now at revision 15677) from the OSGeo4W
> suite on a Windows 7 64-bit machine.
>
> The proj4 string for EPSG:3168 Kertau (RSO) / RSO Malaya (m) from
> Settings->Project Properties is as follows:
>
> +proj=omerc +lat_0=4 +lonc=102.25 +alpha=323.0257905 +k=0.99984
> +x_0=804670.24 +y_0=0 +a=6377295.664 +b=6356094.667915204 +units=m +no_defs
>
> Note in particular:
> +alpha=323.0257905 (azimuth)
> +x_0=804670.24 (false easting)
> +gamma (rectified grid angle)  is not defined
>
> I assigned an SRS for a GeoTIFF raster using GdalTools Translate (Convert
> format), essentially  doing a gdal_translate -a_srs EPSG:3168.
>
> Using GdalTools->Information (gdalinfo) I get this SRS definition:
>
> Driver: GTiff/GeoTIFF
> ...
> Coordinate System is:
> PROJCS["Kertau (RSO) / RSO Malaya (m)",
>     GEOGCS["Kertau (RSO)",
>         DATUM["Kertau_RSO",
>             SPHEROID["Everest 1830 (RSO
> 1969)",6377295.664,300.8017000000015,
>                 AUTHORITY["EPSG","7056"]],
>             AUTHORITY["EPSG","6751"]],
>         PRIMEM["Greenwich",0],
>         UNIT["degree",0.0174532925199433],
>         AUTHORITY["EPSG","4751"]],
>     PROJECTION["Hotine_Oblique_Mercator"],
>     PARAMETER["latitude_of_center",4],
>     PARAMETER["longitude_of_center",102.25],
>     PARAMETER["azimuth",323.0257905],
>     PARAMETER["rectified_grid_angle",323.1301023611111],
>     PARAMETER["scale_factor",0.99984],
>     PARAMETER["false_easting",0],
>     PARAMETER["false_northing",0],
>     UNIT["metre",1,
>         AUTHORITY["EPSG","9001"]],
>     AUTHORITY["EPSG","3168"]]
> ...
>
> Notice that the "false easting" parameter is 0 instead of the correct value
> of 804670.24 as found in proj4. The azimuth value is correct and there is
> also a rectified_grid_angle value.
>
> Looking in OSGeo4W\share\epsg_csv\pcs.csv, I see the following values for
> 3168:
>
> PARAMETER_VALUE_1: 804670.24
> PARAMETER_VALUE_5: 323.01328458
> PARAMETER_VALUE_6: 323.07483685
>
> Now the false easting value is correct, while the azimuth and
> rectified_skew_angle are different.
>
> So where is Gdal in QGis getting it's parameters from?
>
>
>
> *Hilmy*
>
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