I assume that they just buy a zip database, convert zips to longitude and latitude, then plug in the appropriate formula.

Oh, and the US Postal Service sells software for turning addresses into zips. I highly recommend using that rather than trying to roll your own solution. (Even if it does cost money.)

In reply to Re (tilly) 1: Determining the distance from one zip code to another by tilly
in thread Determining the distance from one zip code to another by princepawn

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