[Qgis-user] Custom CRS - adding a rotation

[Paul Paris]

GRASS's v.transform:
https://grass.osgeo.org/grass70/manuals/v.transform.html might also be
useful here.

Paul

On Thu, Apr 6, 2017 at 11:10 AM, Andre Joost <andre+joost at nurfuerspam.de>
wrote:

> 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/q
> uestions/83861/using-customized-coordinate-system-in-arcgis-desktop
>
> HTH,
> André Joost
>
>
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