I have a collection of nine high-resolution map scans illustrating former borders between two nations. The scale is about 1:13,265 for each of the nine images. I want to georeference these maps using QGIS so that I can trace the historical borders, but I want to know whether processing the images - in GIMP, for example - will influence the accuracy of the results.
Right now, each map is on a rectangular sheet with white background. If I crop everything outside the frontier zone and replace the cropped out space with an alpha layer, would this image produce more accurate, less distorted results than the rectangular originals? Using the same ground control points for any given transformation type (e.g., linear, polynomial-1, thin plate spline), would it yield larger or smaller residuals?