By way of example, here's a patch to make the position init code (in fg_init.cxx) cleaner, partly thanks to the FGPositioned changes. It reduces the file size by 200 lines - virtually all of which was copy-and-paste. Once the remaining class (FGAirport) is converted to inherit FGPositioned, all the future patches should be like this - touching one or a few files at most.
This factors the start-offset logic out into a helper, which also does the final property setting (which has to happen on both the /preset and 'real' values). Using the accessors in FGPositioned, and the offset helper, a couple of cases become trivial (fix and nav) and others become much simpler.
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