[OSGeo Africa] Georeferencing and projections

[Hanlie Pretorius]

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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