[OSGeo Africa] Georeferencing and projections

[Hanlie Pretorius]

I tried the suggestion to set the CRS to "South African CRS:HBK_NO_23"
and that seems to work much better.

2014-02-21 18:09 GMT+02:00, Hanlie Pretorius <hanlie.pretorius at gmail.com>:
> Hi Frank,
>
> Do you mean I should change to contents of the .prj file, or should I
> create a custom CRS with k = 1 and assign that to the shapefile?
>
> Thanks
> Hanlie
>
> 2014-02-21 17:37 GMT+02:00, Frank Sokolic <sokolic at worldonline.co.za>:
>> 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
>>> _______________________________________________
>>> Africa mailing list
>>> Africa at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/africa
>>>
>>
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>