Geometry Notes

Pentagon Cells

Every hexagonal DGGS tessellation of a sphere must contain exactly 12 pentagonal cells — a mathematical consequence of Euler's formula for polyhedra. These pentagons sit at the 12 vertices of the underlying icosahedron.

At low resolutions (e.g. resolution 1 with 42 total cells) you will encounter them frequently. A pentagon ring has 5 unique vertices plus a closing repeat (6 total), rather than the 7 of a hexagon. This is correct and expected — not a bug.

const fc = dggs.sequenceNumToGridFeatureCollection([1n], 1);
const ring = fc.features[0].geometry.coordinates[0];
// ring.length === 6 → pentagon (5 unique + 1 closing vertex)
// ring.length === 7 → hexagon (6 unique + 1 closing vertex)

Antimeridian Handling

Cells that straddle the ±180° meridian need special treatment. webdggrid outputs standard GeoJSON with all longitudes in [-180, 180], which is correct for most consumers (Leaflet, Turf.js, PostGIS, etc.).

For MapLibre GL JS / Mapbox GL JS with a globe projection, polygon edges between a vertex at e.g. 170° and one at -170° will be drawn the wrong way around the globe unless you shift the negative longitude into extended range (i.e. -170° → 190°). webdggrid exports unwrapAntimeridianRing for exactly this purpose, and applies it automatically inside sequenceNumToGridFeatureCollection.

import { Webdggrid, unwrapAntimeridianRing } from 'webdggrid';

// sequenceNumToGridFeatureCollection already applies unwrapAntimeridianRing
// internally, so it works out of the box with MapLibre globe.
const fc = dggs.sequenceNumToGridFeatureCollection(seqNums, resolution);
map.getSource('dggrid').setData(fc);

// If you use sequenceNumToGrid directly and need extended coordinates:
const rings = dggs.sequenceNumToGrid(seqNums, resolution);
const corrected = rings.map(unwrapAntimeridianRing);

For renderers that require strictly valid GeoJSON ([-180, 180]), use the raw output of sequenceNumToGrid and handle antimeridian splitting yourself (e.g. with Turf.js booleanCrossesAntimeridian).

Consumer	What to use
MapLibre GL / Mapbox GL globe	sequenceNumToGridFeatureCollection (built-in)
Leaflet, OpenLayers, D3	sequenceNumToGrid — raw [-180, 180]
Turf.js / PostGIS	sequenceNumToGrid — raw [-180, 180]