[Qgis-user] Georeferencing a DXF file

[Gabriel De Luca]

Hi Walt,

  There are some options to georeference a vector layer:

  Since it is 2D, you can georeference it to a projected coordinate system
with ogr2ogr command line application, in the same way that a raster layer
can be georeferenced from control points. You can use any transformation
method and differences will be compensated if the number of control points
provided is higher than the minimum requirement for that method.
  Vector Bender plugin can georeference the layer in QGIS, but if you
define more control points than strictly necessary the transformation will
not be linear also not compensated, but triangulated into as many linear
transformations as control points more than three.
  If you have the coordinates in both systems (the local cartesian of the
original drawing and the geographic but projected one) you can align the
drawing in AutoCAD and export to dxf, import in QGIS and just assign the
proper CRS. Also you can translate, rotate and scale in QGIS with
geoprocessing algorithms or with QAD plugin.

  If you need to georeference to geographic coordinates instead of a
projected system, you can use a linear but 3D transformation to geocentric
and then a conversion to geographic. It's probably not what you want to do. But
you also shouldn't use a 2D linear transformation to go from the drawing
system to a geographic one.

Regards,
Gabriel.

On Wed, Feb 3, 2021 at 4:13 PM Walt Ludwick <walt at valedalama.net> wrote:

> I've received a 3D AutoCAD file from an architect that i managed to
> convert into 2D .dxf format and import into my QGIS project, but it is
> out-of-scale and out-of-place, so i guess i need to add some georeferencing
> coordinates to the file.
>
> This should be easy enough, since it's an accurate representation of a
> building that's already in the project -with an additional wing tacked on-
> so i'm thinking i could just use a few corners of that existing buildings
> as ground control points, and map them to the corresponding corners on this
> .dxf model, then maybe all other co-ordinates could be automatically
> generated by some tool, if i could just find the right one for the job.
>
> Is there in fact any such easy way to do this thing?
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