An interesting variant of space-filling curves + dimensionality reduction is Geohash (https://en.wikipedia.org/wiki/Geohash, http://geohash.org/) which takes a lon/lat and uses a Z-curve approach to produce a hash such as `u4pruydqqvj` representing the location. This hash value is basically "how far along the space-filling curve is the lon/lat located". You're reducing two dimensions (lon/lat) into one dimension (how far along the space-filling curve).

There's a unique side-effect to Geohashes in that the value (`u4pruydqqvj`) can have it's end "lopped off" (i.e. cut down to `u4pru`) and it still represents a less precise, but generally accurate representation of the original lon/lat in most cases (when the curve isn't near the edge of the 2d map!). This allows you to index locations (lat/lon) using a string ('u4pru') which opens you up to doing b-tree, range queries, etc. in traditional database, with one field.

Just a rad math quirk! I'm not an expert, and it's a very dense book, but if someone really wants to get into this kind of stuff the "Bible" is "Foundations of Multidimensional and Metric Data Structures" by Hanan Samet.