Simon Willison’s Weblog

A5 (via) A5 is a new "global, equal-area, millimeter-accurate geospatial index" by Felix Palmer:

It is the pentagonal equivalent of other DGGSs, like S2 or H3, but with higher accuracy and lower distortion.

Effectively it's a way of dividing the entire world into pentagons where each one covers the same physical area (to within a 2% threshold) - like Uber's H3 but a bit weirder and more fun. An A5 reference implementation written in TypeScript is available on GitHub.

This interactive demo helps show how it works:

Why pentagons? Here's what the A5 docs say:

A5 is unique in that it uses a pentagonal tiling of a dodecahedron. [...] The benefit of choosing a dodecahedron is that it is the platonic solid with the lowest vertex curvature, and by this measure it is the most spherical of all the platonic solids. This is key for minimizing cell distortion as the process of projecting a platonic solid onto a sphere involves warping the cell geometry to force the vertex curvature to approach zero. Thus, the lower the original vertex curvature, the less distortion will be introduced by the projection.

I had to look up platonic solids on Wikipedia. There are only five: Tetrahedron, Cube, Octahedron, Dodecahedron and Icosahedron and they can be made using squares, triangles or (in the case of the Dodecahedron) pentagons, making the pentagon the most circle-like option.