[OSGeo Africa] Georeferencing and projections

[Frank Sokolic]

Hi Hanlie,

Have you tried setting the scale factor to +1 instead of -1? So in the 
Layer's coordinate reference change +k=-1 to +k=1 and see if that solves 
your problem. The proj4 string would then be:
+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

You could also set the CRS of the layer to "South African CRS:HBK_NO_23".

Regards, Frank.


On 21/02/2014 16:05, Hanlie Pretorius wrote:
> Hi,
>
> 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:
>
> Extents
> 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:
>
> PROJCS["Hartebeesthoek94_Lo23",GEOGCS["GCS_Hartebeesthoek_1994",DATUM["D_Hartebeesthoek_1994",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",23.0],PARAMETER["Scale_Factor",-1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
>
> Thanks
> Hanlie
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