How to georeference a dxf?

Question

I have got myself a dxf which is built with units in the wrong scale. How do I upscale that in QGIS? Note it does open in QGIS, just in the wrong map units and location.

Answer 1

You georeference rasters and affine vectors :)

QGIS has an affine plug-in (qgsAffine) with which you can scale vector layers. You can also apply x and y offsets to all of the vertices in a vector layer if required. I think you will probably need to convert your DXF to a shapefile to use it though (right-click on the layer in the layers panel and go 'Save as...'). Applying rotation to a vector layer is slightly more difficult.

Answer 2

The fastest way, I think, is to modify your DXF file (scale and place in right coordinates) using a CAD software, and then load it in QGIS.

You can, of course, move your features in QGIS, but you can not scale them. Hope this helps.

Answer 3

I believe this can be accomplished in QGIS .
However I use cad software with my gis software and would do the following in it.
Either way you need some basic information about the dxf.
1. You should know the units of the dxf.
2. You should have a set of known points in the dxf. (at the very least 2 points).

1a. If the original coordinate system (or non-system) is identifiable then you should transform it to a similar known system.
(if the drawing was just drawn in no system but a 0,0 base system assume some type of equal area.) either alambert or albers (dependent upon the coverage area [size])
2a. ensure that all items in the drawing are displayed (no layers turned off).
3a. select all the features.
4a. move all the features from a known point to a known point.
5a. after determining the original scale resize all selected objects empirically
6a. rotate all selected objects empirically.
Save and/or export the selected objects with a known CRS.

Answer 4

nhopton's answer above on using the qgsAffine plugin worked for me only last week. I had a dxf that was scaled 1000x the CRS I wanted. I transformed the dxf to shp with the standard plugin, then transformed X and Y by multiplying by .001 in qgsAffine, and problem was solved.

PS Belated thanks to nhopton for the advice, which I didn't get to extend before the QGIS Forum was shut down!