[Qgis-user] Custom CRS - adding a rotation

[Andre Joost]

Am 05.04.2017 um 20:46 schrieb Brock Riedell:
> I have some downloaded geologic map shapefiles that were digitized
> incorrectly in UTM space, and they render a few hundred meters away from
> their proper locations on georeferenced U.S. topo maps as well as on
> Google, Bing, and ESRI online base map data. This is too much for datum
> shift; the NAD27 to NAD83 transformation should offset coordinates about
> 90 m.  I have determined that the correction involves a counterclockwise
> rotation of ~1.4 degrees and not a simple translation. I have reviewed
> the PROJ.4 documentation about using the 7-parameter 'towgs84' options
> to add the rotation.  I have not been able to get this working by trial
> and error; the data plot out 150-200 km away from where they should be.
> Questions:
>
> (1) Do I need to input the rotation value as both Rx and Ry?
> (2)The documentation says rotations are input as seconds of arc rather
> than degrees, correct?
> (3) How does the scaling parameter in towgs84 interact with the scale
> factor k_0?

The towgs84 parameters are really ment for small rotations only. If you 
have big rotations, think of using a custom oblique mercator transformation.

You can find an example at 
http://gis.stackexchange.com/questions/83861/using-customized-coordinate-system-in-arcgis-desktop

HTH,
André Joost