Add Cell.ToLatLng for H3 cell to geographic coordinate conversion#16
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Add adjustOverageClassII, faceNeighbors table, maxDimByCIIres, and unitScaleByCIIres. Ported line-by-line from the H3 C reference implementation (faceijk.c). These are prerequisites for Cell.ToLatLng which needs to handle cells near icosahedral face boundaries.
Add the reverse H3 path: Cell → FaceIJK → hex2d → LatLng. - hex2dToGeo: reverse gnomonic projection (hex2d + face → LatLng) - cellToFaceIjk: combines baseCellToFaceIjk + toFaceIjkWithInitializedFijk + adjustOverageClassII, with full pentagon handling - Cell.ToLatLng(): public API returning cell center as LatLng All functions ported from the H3 C reference implementation. Test coverage: - 12 ground-truth values verified against h3-python - All 12 pentagon base cells at res 0 - All 122 base cells round-trip at res 0 - 13 global cities × 8 resolutions (104 round-trip tests) - Invalid cell error handling
iandees
approved these changes
Mar 30, 2026
jrhy
marked this pull request as ready for review
March 30, 2026 16:21
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Motivation and Context
The
Cell.ToLatLng()method (reverse ofNewCellFromLatLng) completes theCell ↔ LatLng round-trip. Downstream consumers need this to convert H3 cell
indices back to geographic coordinates for distance and centroid computations.
What is changed?
Adds three functions to complete the reverse H3 path:
adjustOverageClassII(overage.go) — handles cells near icosahedral face boundariesby translating FaceIJK coordinates to the correct neighboring face. Includes
faceNeighborstable,maxDimByCIIres, andunitScaleByCIIres.hex2dToGeo(coordijk.go) — reverse gnomonic projection (hex2d → LatLng)Cell.ToLatLng()(cell.go) — public API tying it all together viacellToFaceIjkwith full pentagon handling (leading digit 5 rotation,leading digit 4 special overage, secondary overage loop)
All ported line-by-line from the H3 C reference implementation (
faceijk.c,h3Index.c).Testing
hex2dToGeo↔geoToHex2dround-trip across faces