The H3 system assigns a unique hierarchical index to each cell. The H3 index of a resolution r cell begins with the appropriate resolution 0 base cell number. This is followed by a sequence of r digits 0-6, where each i^{th} digit d_{i} specifies one of the 7 cells centered on the cell indicated by the coarser resolution digits d_{1} through d_{i-1}. A local hexagon coordinate system is assigned to each of the resolution 0 base cells and is used to orient all hierarchical indexing child cells of that base cell. The assignment of digits 0-6 at each resolution uses a Central Place Indexing arrangement (see Sahr, 2014). In the case of the 12 pentagonal cells the indexing hierarchy produced by sub-digit 1 is removed at all resolutions.
Child hexagons are linearly smaller than their parent hexagons.
The H3Index is the integer representation of an H3 index, which can be placed into multiple modes to indicate the concept being indexed.
Mode 0 is reserved and indicates an invalid H3 index.
Mode 0 contains a special index, H3_NULL
, which is unique: it is bit-equivalent to 0
.
This index indicates, specifically, an invalid, missing, or uninitialized H3 index;
it is analogous to NaN
in floating point.
It should be used instead of an arbitrary Mode 0 index, due to its uniqueness and easy identifiability.
The components of the H3 cell index (mode 1) are packed into a 64-bit integer in order, highest bit first, as follows:
The components of the H3 unidirectional edge index (mode 2) are packed into a 64-bit integer in order, highest bit first, as follows:
The canonical string representation of an H3Index is the hexadecimal representation of the integer, using lowercase letters. The string representation is variable length (no zero padding) and is not prefixed or suffixed.
The three bits for each unused digit are set to 7.
The layout of an H3Index is shown below in table form. The interpretation of the "Reserved/edge" field differs depending on the mode of the index.
0x0F | 0x0E | 0x0D | 0x0C | 0x0B | 0x0A | 0x09 | 0x08 | 0x07 | 0x06 | 0x05 | 0x04 | 0x03 | 0x02 | 0x01 | 0x00 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0x30 | Reserved | Mode | Reserved/edge | Resolution | Base cell | |||||||||||
0x20 | Base cell | Digit 1 | Digit 2 | Digit 3 | Digit 4 | Digit 5 | ||||||||||
0x10 | Digit 5 | Digit 6 | Digit 7 | Digit 8 | Digit 9 | Digit 10 | ||||||||||
0x00 | Digit 10 | Digit 11 | Digit 12 | Digit 13 | Digit 14 | Digit 15 |