H3 grid

The H3 grid is a spatial indexing system, developed by Uber, that divides the earth’s surface into a hierarchy of hexagonal cells at multiple resolutions. Every cell has an identifier, and cells nest inside coarser cells, so the same data can be summarised at different scales without being recalculated from scratch.

Hexagons rather than squares, for one practical reason: every neighbour of a hexagon shares an edge and sits the same distance away. A square grid has eight neighbours at two different distances, four sharing an edge and four only a corner, which distorts anything that spreads outward, like travel time.

Resolution is described by the edge length of the cell. At resolution 3 the cells are around 69 kilometres across the edge, which suits countries and large regions. At resolution 10 they are around 75 metres, which suits a neighbourhood analysis. Choosing the resolution is a choice about how fine the result can be and how long it takes to compute. Both figures come from Uber’s own documentation, and GOAT’s accessibility heatmaps use the grid at neighbourhood resolutions.