[OSGeo Africa] Georeferencing and projections

Hanlie Pretorius hanlie.pretorius at gmail.com
Fri Feb 21 06:05:37 PST 2014


I'm working (mostly) in QGIS 2.0.1 and Windows 7.

I received a CAD drawing using the GIS version of the SACS Lo 23 (i.e.
x and y are swopped and all coordinates are muliplied by mins 1) from
a surveyor and I followed the process below to get a shapefile out of
some of the data:

1. (In Arcmap because QGIS wouldn't read the DWG) Set up a layer with
Hartbeeshoek Lo23 as the coordinate system.

2. Load the DWG file.

3. Select some entities and export them to a shapefile.

4. Define the projection of the exported shapefile as Hartbeeshoek Lo23.

When I load this shapefile into QGIS, I get the following metadata:

In layer spatial reference system units
xMin,yMin 671.115,-3029070.13 : xMax,yMax 16804,-3027273.90
Layer Spatial Reference System
+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

Everything seems fine up to now, but when I try to load this shapefile
file into any project that does not have the same reference system as
the file (and with on-the-fly projection enabled), the entities in the
shapefile file end up in the wrong place. When I use the same
projection for the project and the shapefile, it ends up in the
correct place, as tested against other shapefiles for the same area
that I have in the same projection.

I have tested the on the fly projection with two other shapefiles
files that someone sent me in two different projections and they land
exactly on top of one another.

Does anyone know what could be going wrong with the file that I
generated from the CAD?

The contents of the shapefile's .prj file is:



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