Just making a note on a geo-referencing task I was doing with QGIS.
I had an outer boundary shape of a village, and had to geo-reference a PDF map of that village which has some internal data that I wanted. (won't go into more details here, but see this and this to get your shape and map.)
I looked up the QGIS documentation : http://docs.qgis.org/2.8/en/docs/user_manual/plugins/plugins_georeferencer.html#available-transformation-algorithms
And chose the "Polynomial 2" tranformation type.
My georeferencing points (GCP) were points along the outer boundary of the village only : nothing from inside / center.
Ran the georeferencer : it got done instantly, and lo and behold, the image/raster map appeared perfectly on my shapefile map. It's a perfect fit, and the original map doesn't have any major distortions.. it has a slight rotation.
The output .TIF file is also much smaller than what I've had earlier : only 6.5MB compared to 405MB for an earlier run.
Previously based on some other article I had been using the setting "thin plate spline". It was extremely slow and created very distorted outputs and much larger .tif's . So for my kind of task (outer boundaries pinning), the "Polynomial 2" transformation has proved to be much better on all accounts.