The Geo API has been around for a while, appearing in the Redis unstable branch about ten months ago and that was, in turn, based on work from 2014. There’s a bit of history in that development process, which being practical folk we’ll skip past and go straight to the stuff that makes your development day better.
At its simplest, the GEO API for Redis reduces longitude/latitude down into a geohash. Geohash is a technique developed in 2008 to represent locations with short string codes. The Geohash of a particular location, say Big Ben in London, would come out as “gcpuvpmm3f0” which is easier to pass around than “latitude 51.500 longitude -0.12455”. The longer the string, the more precise the geohash code.
That encoding into a string is good for humans and URLs but it isn’t particularly space efficient. The good news is geohashes can be encoded as binary and using 52 bits, a geohash gets down to 0.6 meter accuracy which is good enough for most uses. A 52-bit value which just happens to be able to be a small-enough integer to live in a Redis floating-point double safely and that’s what the Geo API works with behind the scenes.