[Qgis-user] Custom CRS - adding a rotation

[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?


Thanks and regards
Brock


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