Cell mode

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<sup>th</sup> digit d<sub>i</sub> specifies one of the 7 cells centered on the cell indicated by the coarser resolution digits d<sub>1</sub> through d<sub>i-1</sub>. 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.

H3 Cell Index

An H3 Cell index (mode 1) represents a cell (hexagon or pentagon) in the H3 grid system at a particular resolution. The components of the H3 Cell index are packed into a 64-bit integer in order, highest bit first, as follows:

1 bit reserved and set to 0,
4 bits to indicate the H3 Cell index mode,
3 bits reserved and set to 0,
4 bits to indicate the cell resolution 0-15,
7 bits to indicate the base cell 0-121,
3 bits to indicate each subsequent digit 0-6 from resolution 1 up to the resolution of the cell (45 bits total are reserved for resolutions 1-15)

The three bits for each unused digit are set to 7.

Bit layout of H3Index for cells

The layout of an H3Index is shown below in table form. The interpretation of the "Mode-Dependent" field differs depending on the mode of the index.

<table> <tr> <th></th> <th>0x0F</th> <th>0x0E</th> <th>0x0D</th> <th>0x0C</th> <th>0x0B</th> <th>0x0A</th> <th>0x09</th> <th>0x08</th> <th>0x07</th> <th>0x06</th> <th>0x05</th> <th>0x04</th> <th>0x03</th> <th>0x02</th> <th>0x01</th> <th>0x00</th> </tr> <tr> <th>0x30</th> <td>Reserved (0)</td> <td colspan="4">Mode (1)</td> <td colspan="3">Reserved (0)</td> <td colspan="4">Resolution</td> <td colspan="4">Base cell</td> </tr> <tr> <th>0x20</th> <td colspan="3">Base cell</td> <td colspan="3">Digit 1</td> <td colspan="3">Digit 2</td> <td colspan="3">Digit 3</td> <td colspan="3">Digit 4</td> <td>Digit 5</td> </tr> <tr> <th>0x10</th> <td colspan="2">Digit 5</td> <td colspan="3">Digit 6</td> <td colspan="3">Digit 7</td> <td colspan="3">Digit 8</td> <td colspan="3">Digit 9</td> <td colspan="2">Digit 10</td> </tr> <tr> <th>0x00</th> <td>Digit 10</td> <td colspan="3">Digit 11</td> <td colspan="3">Digit 12</td> <td colspan="3">Digit 13</td> <td colspan="3">Digit 14</td> <td colspan="3">Digit 15</td> </tr> </table>

H3 Cell Index

Bit layout of H3Index for cells

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