You're right. I suppose either GPS co-ordinates or traditional longitude and latitude would be more than sufficient. It's just getting the data -- the zip codes and postcodes (and any others that other countries use) to correlate with either of the two aforementioned.

I don't think it's going to be easy!

However, once you have that data, the zip codes and postcodes become one (not in a Buddhist sense) and become irrelevant insofar as the algorithm is concerned. Those codes are used to retrieve L&L or GPS co-ordinates and the system uses those to calculate distance.

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.