[Qgis-user] Custom CRS - adding a rotation

[Fernando M. Roxo da Motta]

On Wed, 5 Apr 2017 11:46:57 -0700, Brock Riedell <kbrcbc at shaw.ca> wrote:


> 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

  It seems that it fits completely the definition of an affine
transformation.   There is a plugin with this name that should do the
magic.


  HTH


> 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
> 
> 








  Roxo

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