We got geocoding out of the way in the first part, so now it's time to talk distances. To figure out how far apart two places are, we could use the traditional method of calculating distances between two points, but that damn Columbus didn't fall off into the Twighlight Zone when he passed the horizon. Turns out the Earth isn't flat, go figure. Oh, and since it isn't flat, assuming that it is will cause your numbers to be off when calculating distances using normal geometry-sometimes by miles.

He links to a formula for finding the difference when the distances are short as well as a PHP implementation of the Haversine formula when the distances get long.

He combines this new knowledge with the information already obtained by the geocoding in the first part and a SQL database to help narrow down the items in the list. Then, for each of those, he compares the latitude and longitudes to get the difference between them (in miles).