Conversion from latitude/longitude to containing H3 cell index

This operation is performed by function geoToH3. See the comments in the function for more detail.

The conversion is performed as a series of coordinate system conversions described below. See the page Coordinate Systems used by the H3 Core Library for more information on each of these coordinate systems.

1. The input latitude/longitude coordinate is first converted into the containing icosahedron face and a Hex2d coordinate on that face using function _geoToHex2d, which determines the correct face and then performs a face-centered gnomonic projection into face-centered polar coordinates. These polar coordinates are then scaled appropriately to a Hex2d coordinate on the input grid resolution r.

2. The Hex2d coordinate is converted into resolution r normalized ijk coordinates using function _hex2dToCoordIJK.

3. The face and face-centered ijk coordinates are then converted into an H3Index representation using the following steps:

   • the H3 index digits are calculated from resolution r up to 0, adjusting the ijk coordinates at each successively coarser resolution.
   • when resolution 0 is reached, if the remaining ijk coordinates are (0,0,0) then the base cell centered on the face is chosen for the index
   • if the remaining resolution 0 ijk coordinates are not (0,0,0), then a lookup operation is performed to find the appropriate base cell and the required rotation (if any) to orient the cell in that base cell's coordinate system. The index is then translated and rotated into the coordinate system centered on the new base cell.