Terminology

The following are technical terms used by H3.

• H3Index:
  • an unsigned 64-bit integer representing any H3 object (hexagon, pentagon, directed edge ...)
  • often represented as a 15-character (or 16-character) hexadecimal string, like '8928308280fffff'
  • the full term "H3 index" should be used to avoid confusion with other common uses of "index"; when a "traditional" index is needed, prefer using "number", "pos", or another term to avoid confusion
• mode:
  • an integer describing the type of object being represented by an H3 index
  • this integer is encoded in the H3Index
• cell or H3 cell:
  • a geometric/geographic unit polygon in the H3 grid, corresponding to an H3Index of mode 1 (hexagon or pentagon)
  • for functions that can handle either hexagons or pentagons, the more general term "cell" should be used whenever possible
• hexagon:
  • an H3 cell that is a topological hexagon
  • below, we explain that functions that only work with hexagons have an Unsafe suffix; these functions are paired with ones having a Safe suffix, meaning they can handle pentagons, but are slower
• pentagon:
  • an H3 cell that is a topological pentagon
• directed edge:
  • represents a traversal from an origin cell to an adjacent destination cell
  • corresponds to an H3Index of mode 2
• grid:
  • the graph with nodes corresponding to H3 cells, and edges given by pairs of adjacent cells
  • for example, gridDistance is the minimal number of edges in a graph path connecting two cells
• lat/lng point:
  • a representation of a geographic point in terms of a latitude/longitude pair
  • when abbreviating, we use "lng" (instead of "lon") for longitude
• topological:
  • H3 cells are topological pentagons or hexagons, in the sense that they have 5 or 6 neighbors, respectively, in the H3 grid
  • the majority of hexagons are also geometric hexagons (similarly with pentagons), in that they have 6 edges and vertices when represented as polygons of lat/lng points
  • a small number of hexagons are not geometric hexagons (similarly with pentagons), in that they have extra vertices and edges due to distortion around icosahedron boundaries
  • for more details, see this h3-js issue or this Observable post
• base cell:
  • one of the 122 H3 cells (110 hexagons and 12 pentagons) of resolution 0
  • every other cell in H3 is a child of a base cell
  • each base cell has a "base cell number" (0--121), which is encoded into the H3Index representation of every H3 cell
  • there is a one-to-one correspondence between the "base cell number" and the H3Index representation of resolution 0 cells
    • e.g., base cell 0 has H3Index hexadecimal representation '8001fffffffffff'
• boundary:
  • all or part of the list of geometric points that enclose an H3 cell
  • may include more than 6 points in the case that a cell is not a geometric hexagon, such as when a hexagon crosses an icosahedron boundary
  • may also be used to describe the boundary between two geometric cells, as in the case of an edge
  • represented in the H3 codebase with the CellBoundary struct (previously GeoBoundary before v4.0)
• H3_NULL:
  • equivalent to 0 and guaranteed to never be a valid H3Index (even after any future H3 modes are added)
  • returned by functions to denote an error, or to denote missing data in arrays of H3Index
  • analogous to NaN in floating point

Use of "hex", "hexagon", "cell", "pentagon", etc.

We realize that "hex" or "hexagon" will still be used informally to refer to the concept of "cell" (As the development team, we do it ourselves!). This should be expected in casual, informal discussions of H3. However, when precision is required, we advise the use of strict technical terms like "index", "cell", "hexagon", "pentagon", etc. In the codebase and in the documentation, strictly correct terminology should always be used, as many functions and algorithms distinguish between hexagons and pentagons.

References

• Technical RFC for naming concepts