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I am looking to overlay a PNG map image on top of a mapping service such as MapBox or Google Maps. The map PNG is a custom design to suit the application but is not geographically accurate (not too far out, but is not a simple shift or resize etc). The issue is when trying to show the user's location on top of this map, it would show the location pin correctly on the accurate map, but would not appear correctly on the PNG map.

To get the map to line up, I have previously used QGIS to best align the maps.

My understanding is that it could perhaps be solved by applying some sort of skew algorithm on the user's coordinates to match up with the PNG map. Does QGIS offer a solution to this, or is there an existing algorithm or program to help with this?

I imagine that the solution would allow a user to select reference points on both maps to build a list of coordinate mappings, and then would generate a function that would take the user's coordinates and output coordinates that fit on the PNG map.

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    it might be simpler to do the opposite, i.e. to properly georeference the png using control points and to keep GPS locations intact
    – JGH
    Commented Dec 10, 2021 at 21:25

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Since you appear to have already used the georeferencer in QGIS I'd recommend to make use of the transormation type 'Thin Plate Spline'. This allows very local skewing without distorting the original image too much (or at all when cleverly done).

Manipulating coordinates simply to fit a custom image overlay seems like putting the cart before the horse to me tbh.

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