related to https://github.com/gbif/occurrence/issues/280

// based on https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#ECMAScript_.28JavaScript.2FActionScript.2C_etc..29 // but the floor rounding removed. I did so with the assumption, that the rounded part corresponded to the tile coordinates var Geohash = require("latlon-geohash@1.1.0"); function lon2tile(lon,zoom) { return (lon+180)/360*Math.pow(2,zoom); } function lat2tile(lat,zoom) { return (1-Math.log(Math.tan(lat*Math.PI/180) + 1/Math.cos(lat*Math.PI/180))/Math.PI)/2 *Math.pow(2,zoom); } function tile2long(x,z) { return (x/Math.pow(2,z)*360-180); } function tile2lat(y,z) { var n=Math.PI-2*Math.PI*y/Math.pow(2,z); return (180/Math.PI*Math.atan(0.5*(Math.exp(n)-Math.exp(-n)))); } function getTileCoordinates(lat, lon, zoom, extend) { let tx = lon2tile(lon, zoom), ty = lat2tile(lat, zoom); let result = { x: extend * (tx % 1), y: extend * (ty % 1), tileX: Math.floor(tx), tileY: Math.floor(ty) }; return result; } const extend = 512; const precision = 3; const zoom = 2; const lat = 63.252872; const lon = 34.123535; const initGeoHash = Geohash.encode(lat, lon, precision); console.log(`initial geohash: ${initGeoHash}`); const {tileX, tileY, x, y} = getTileCoordinates(lat, lon, zoom, extend); const localTileX = Math.floor(x); const localTileY = Math.floor(y); // Once we know that this has fallen into the northern geohash, then we can simple +1 to this and it will be correct console.log(`local tile x: ${localTileX}`); console.log(`local tile y: ${localTileY}`); const reLon = tile2long(tileX + localTileX/extend, zoom); const reLat = tile2lat(tileY + localTileY/extend, zoom); console.log(`reLon: ${reLon}`); console.log(`reLat: ${reLat}`); console.log(`new geohash: ${Geohash.encode(reLat, reLon, precision)}`); console.log(Geohash.neighbours(initGeoHash)); // from this we can see that it is the northern geohash so we should move localeTileY down (+1) // in this example we round the coordinates in such a way that the geohash moves from ug0 => ug2