Georeferencing CAD drawing and setting ground control points in GRASS?

Question

I'm fairly new to QGIS and Grass GIS, and learning the interface.

I have a CAD drawing that has no coordinate system assigned and needs to be georeferenced to NAD83 HARN Northern Colorado (feet).

I'm trying to use v.rectify to match known control points on the CAD drawing to their counterparts on a properly georeferenced vector map downloaded from the county website.

I keep getting the error:

Poorly placed control points. Can not generate the transformation equation.

So I assume I'm not going about setting these points properly.

I'm using the Digitize New Points function in GRASS to get the source coordinates at road intersections on the CAD drawing and in the County data, then putting them in notepad and using that for my input file.

Answer 1

Try using the Georeferencer in QGIS. This tool allows you to enter the control points by clicking on the CAD drawing and on the map canvas. It seems more user-friendly than the process you're trying to use.

1. Add a basemap to a QGIS project. An easy way to do this is with the QuickMapServices plugin.

2. Add the vector layers from the county into QGIS. Make sure they are in the correct location by comparing them to the basemap. (This was the reason for adding the basemap - you can now remove the basemap if you want.)

3. Enable snapping at intersections. If you're using QGIS 2.18 or older, snapping is available through the Settings menu. In QGIS 3.0 and up, snapping is available through the Snapping toolbar.

4. Install and enable the Georeferencer plugin. Plugin menu > manage and install plugins > search for Georeferencer and check the tick box next to its name

5. Open the Georeferencer plugin and load in your CAD drawing. Choose NAD83 HARN Northern Colorado (feet) as the CRS.

6. To add a control point, click the Add control points button, then click on the CAD drawing, choose the option from map canvas then click on the corresponding point on the map canvas. Snapping will make it easy to set your ground control points exactly at the road intersections.

7. Once you've added all your control points, choose a transformation type and click the green button to georeference the drawing. If you're not satisfied with the result, you can add or change your control points, or try a different transformation type.

   For more detailed instructions on using the Georeferencer, see the QGIS User Guide.

Answer 2

The georeferencer now supports vector georeferencing, that would be the easiest way to solve this in the future.