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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?

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    One question per Question, please. – Vince May 17 '17 at 22:29
  • edited to remove the related question i asked about whether file size is also affected. – grad student May 18 '17 at 2:04
  • Please take the Tour to better understand how GIS SE works. You can vote to reopen after you've made edits (commenting that you've made edits doesn't actually change the state of the hold). – Vince May 18 '17 at 2:12
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If you have a very large map and you want to georeference only a small portion of it, it would make sense to crop it, as you will have less to georeference, less error, less area. For example, if one would have a world map and needs only New Zealand, to georeference it you would have to place points on all the map; from my point of view it makes sens to crop the unwanted bits, as long as it would not influence the end result. Also cropping a large map, would help on the size part, meaning it's easier for ArcMap(QGIS) to draw a 100 Mb file, then an 1 Gb file.

I don't see how would an alpha channel improve your accuracy, when it only has a role in transparency.

So to conclude, yes on cropping, it would make your file size smaller and easier to handle, alpha has no influence in gereoferencing afaik, try to place your points as accurate as possible, try all the transformation before settling for the one that fits your needs the best.

Here is a great post with some good practical advice on georeferencing.

  • "...as long as it would not influence the end result." <-- this is exactly what my question is asking... also, there is no difference in how qgis and arcgis handle this? – grad student May 18 '17 at 15:23
  • The math implied in the transformations is the same, even if the UI is different. A 2nd degree polynomial is the same in QGIS and ArcGIS. To be sure you get the best georeferencing, try on a cropped image and on a full image and see which one is more close to reality. – alecsx May 19 '17 at 16:18

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