How many reference points should I define?
It largely depends on your image. The closest to reality your image is, the less point you will need.
Consider the following two scenarios :
If your image is very accurate, 3 reference points are enough. For an optimal accuracy, choose points located on the edges of your map image, as illustrated below.
On the contrary, if your image is not very accurate (contains deformations), you will need more than 3 points. For highly deformed maps such as ski resorts, it is normal to have up to 80 points!
Our recommandation is to start with few points, use the test mode to validate your work, and add reference points until you are satisfied with the achieved precision.