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I'm working on georeferencing old maps for archaeological purpose. Of course, colonial maps all do have some degree of transformation needed. But sometimes a specific region is well mapped while the neighbouring one is not. So I wonder if there is a way to calculate the amount of (in)accuracy, or the level to which the original image has been transformed in order to fit the actual landscape? Could a new raster be generated with these values? I'm using QGIS and TPS for georeferencing, but it can be on any platform.

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    as far as i remember, the accuracy is normally calculated from the ground control points, not the image itself. i dont know what qgis plugin you are using, but it seems the docs.qgis.org/2.8/en/docs/user_manual/plugins/… includes the residual as well as the mean error for the GCPs (figure 1) – Jonas Aug 21 '18 at 11:22
  • So the overall accuracy is expressed by the global mean error, and each GCP has its own residual. I could compare two maps with the same control points, and see which one is more or less accurate. But how could I use these values to analyze a single map? Split it into tiles maybe? – gvanhavre Aug 23 '18 at 9:51
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I'm not sure if this is what you want but when you load a map with 4 or more calibration points in MAPC2MAPC, it calculates a best-fit affine transformation between coordinates and pixels - and then reports on how good the fit is.

If you have an image and a suitable calibration file it will only take seconds.

But what to do if the calibration is 'off' is another matter!

http://www.the-thorns.org.uk/mapping/help/errors.html

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    You should add that you are the author of the program. – AndreJ Sep 23 '18 at 11:40
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Load the maps shapefile from a trusted source and compare them. If there is some error it can be further improved by taking more control points, changing transformation parameters and trying a different datum.

  • @Damini Jain, I think it is not that easy to find archaeological shapefiles from a trusted source. I assume it could more efficient to find archaeological web map service (WMS, TMS/WMTS etc.) and use it as a background map. – Taras Aug 21 '18 at 4:49
  • There are many sites of government and sources from where you can get it. You have to look on the Internet. So my answer isn't wrong. – Damini Jain Aug 21 '18 at 6:00
  • @DaminiJain, thanks, but georeferencing itself is not the problem. I'm looking for a way to evaluate the degree of transformation of the original image, like on a raster calculation. If it matches archaeological sites or not will be a question for later. – gvanhavre Aug 21 '18 at 9:13
  • @gvanhavre geog.uoregon.edu/amarcus/geog418/Labs/Lab06_rectification.htm hope this helps. – Damini Jain Aug 21 '18 at 10:15

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