[OSGeo Africa] QGIS and projection names

[Tim Sutton]

Hi Hanlie


On Thu, Apr 17, 2014 at 2:03 PM, Hanlie Pretorius <bloupanda at gmail.com>wrote:

> Hi,
>
> I created a shapefile in QGIS and selected South African CRS:
> HBK_NO_23 as my CRS.
>
> When I open the file in QGIS, the metadata for the projection reads:
>
> USER:100001 -  * Generated CRS (+proj=tmerc +lat_0=0 +lon_0=23 +k=1
> +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs)
>
> Furthermore, this 'user specified' CRS now exists in my list of
> projections. I find this rather annoying because I end up with a
> growing list of identical projection definitions.
>

If the growing list is of identical definitions then it is a bug - please
file a ticket at:

http://hub.qgis.org


>
> The .prj file reads:
>
>
> PROJCS["Transverse_Mercator",GEOGCS["GCS_WGS_1984",DATUM["D_unknown",SPHEROID["WGS84",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",23],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]
>
> and the .qpj file reads:
>
> PROJCS["unnamed",GEOGCS["WGS
>
> 84",DATUM["unknown",SPHEROID["WGS84",6378137,298.257223563],TOWGS84[0,0,0,0,0,0,0]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",23],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]
>
>
> I'm just wondering if all these different CRS specifications may be
> causing problems when using the shapefile with other programs, such as
> ArcGIS. I am having problems with shapefiles looking correct in QGIS
> but shifted in ArcGIS. The shift is about 70 meters in the x direction
> and 255 meters in the y direction.
>
> I found this web page (http://www.georeference.org/forum/t115330)
> saying that the difference between WGS84 and Hartbeeshoek 1994) is
> approximately 0.2 m and 0.3 m in Lo y and x coordinates respectively.
>
> I see in ArcGIS the transformation reads dx=0, dy=0, dz=0. So if the
> two datums are identical for most purposes, what else could be causing
> a shift in positions between the two programs?
>

What happens if you remove the towgs parameter from the qpj file? Does the
shift change?

See http://proj.maptools.org/gen_parms.html#towgs84 for more info on what
that option is doing.

Regards

Tim


> Thanks
> Magriet
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-- 
Tim Sutton
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