If you look at a large number of historical maps, you’ll notice that each mapmaker / each country had their own errors (sometimes trivial spelling mistakes, or

If you look at a large number of historical maps, you’ll notice that each mapmaker / each country had their own errors (sometimes trivial spelling mistakes, or pairs of cities with the labels swapped, other times comically wrong coastlines or even stuff like California-as-an-island) which persisted for centuries after corrections had been made in maps from other makers. If you compare the errors you can figure out pretty easily which previous map “lineage” was used as the basis for any particular map.