[OSGeo Africa] QGIS and projection names

[Hanlie Pretorius]

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.

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?

Thanks
Magriet