I am georeferencing some old aerial pictures in QGIS. The area encompass mountains, hills and a flat area.
For some pictures the error is quite low and I am satisfied with the overlapping (error is around 2 using the polynomial 2). For others, the error is very high (as much as 100!). What bothers me most, is that I cannot understand the red lines that appear in the screen. When I delate these points with long lines, sometimes the error decreases. Other times the nearby points increase even more the lengths of these lines.
I am using as SR the 3003 and as method for transformation polynomial 1 or 2 or 3 and nearest point. I usually put more than 30 points of control.