In this picture we can examine a method for raster data rectification. In the lower left window of each one, we  have the red vector "coverage" (island of Salamina, near Piraeus), that is established in a certain projection  system & its elements have "real" coordinates. The black & white (upper of it) picture is a raster (scanned) image of the same area & we want to transform it from its ‘paperβ coordinates to the real ones. Adjusting frames in these two small windows, we define the areas we want to overlay & then we put the first 4 links (green arrows in the right window of the 1st picture). After the first registration (which brings us to the 2nd picture), we put more links to achieve better transformation, trying also to have low RMS values, by deleting and keeping the most proper ones (see the two upper windows).