I am a beginner in geomatics.
I am trying to georeference some images in QGIS.
I would like to understand what is happening behind the different transformation types (linear, polynomial, Helmert...). What I don't understand is the red lines that appear after inserting a GCP. These lines take as a starting point the point the user put, but I don't understand their orientation.
Does this show a gap between the coordinates given by the user and the coordinates of the map?
Example : http://hpics.li/dbe04f9