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
Best Answer
The following are quoted from the QGIS docs regarding the Available Transformation algorithms:
In terms of the red lines , these are likely be georeferencing errors which can show the residual difference between various sets of map control points.