[OSGeo Africa] QGIS and projection names

[Aslam Parker]

Hi all

Practically, the only way , (unless you survey from the IGS global
network) to determine a coordinate accurate to a few cm in South Africa
is to determine the position relative to a known point in the national
control survey network (trig beacon, town survey marks or Trignet,) the
coordinates of which are all reported in Hartebeesthoek 94 datum.


We need to distinguish between the WGS ellipsoidal, which is common to
Hartebeesthoek 94 datum as we'll as the WGS 84 datum/ reference frame.
The WGS84 datum is constantly realigned to the latest realization of the
International Terrestrial Reference Frame (ITRF).


For all practical purposes, WGS 84 datum and Hartebeesthoek 94 is
identical. For this reason a CRS prefixed with HBK, which is short for
Hartebeesthoek 94, indicates that there is zero geocentric translation
to WGS 84. This technically incorrect zero translation does not
generally have any implication,  unless u combine data actually recorded
at similar precision in the WGS 84 reference frame with Hart 94 data.
This is highly unlikely in precise applications. In less precise data
collection, such as use of autonomous GPS, the uncertainty in coordinate
is greater than the difference between the two reference frames and it
does not matter.

However for precise geodetic applications, this difference is very
significant, but then again one would not use GIS to transform data in
these applications.

So to summarize, it is no problem having multiple CRS with the same
definitions, as the GIS will apply a zero transformation to the various
data sets and it should be consistent, as long as it was collected to
the same accuracy.


The 70m in y 255m in x is typical of data that was referenced in the
Cape Datum ( mod Clarke 1880) ellipsoidal, and not a Hart94 - WGS 84
difference., in that case you could have brought in historical data
under the incorrect assumption that it was in Hart94.

I hope this advice helps.



>>> Hanlie Pretorius <bloupanda at gmail.com> 04/17/14 15:20 PM >>>
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
MAfrica at lists.osgeo.org
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