geomodel.js
This library is an implementation of the Geomodel/Geocell concept.
A geocell is a hexadecimal string that defines a two dimensional rectangular region inside the [-90,90] x [-180,180] latitude/longitude space. A geocell's 'resolution' is its length. For most practical purposes, at high resolutions, geocells can be treated as single points.
Much like geohashes (see http://en.wikipedia.org/wiki/Geohash), geocells are hierarchical, in that any prefix of a geocell is considered its ancestor, with geocell[:-1] being geocell's immediate parent cell.
To calculate the rectangle of a given geocell string, first divide the [-90,90] x [-180,180] latitude/longitude space evenly into a 4x4 grid like so:
+---+---+---+---+ (90, 180) | a | b | e | f | +---+---+---+---+ | 8 | 9 | c | d | +---+---+---+---+ | 2 | 3 | 6 | 7 | +---+---+---+---+ | 0 | 1 | 4 | 5 | (-90,-180) +---+---+---+---+
NOTE: The point (0, 0) is at the intersection of grid cells 3, 6, 9 and c. And, for example, cell 7 should be the sub-rectangle from (-45, 90) to (0, 180).
Calculate the sub-rectangle for the first character of the geocell string and re-divide this sub-rectangle into another 4x4 grid. For example, if the geocell string is '78a', we will re-divide the sub-rectangle like so:
. . . . . . +----+----+----+----+ (0, 180) | 7a | 7b | 7e | 7f | +----+----+----+----+ | 78 | 79 | 7c | 7d | +----+----+----+----+ | 72 | 73 | 76 | 77 | +----+----+----+----+ | 70 | 71 | 74 | 75 | . . (-45,90) +----+----+----+----+ . . . .
Continue to re-divide into sub-rectangles and 4x4 grids until the entire geocell string has been exhausted. The final sub-rectangle is the rectangular region for the geocell.
Usage
Create a Geomodel instance:
var geomodel = require('geomodel').create_geomodel();
var hash = geomodel.compute_hash(lat, lon, hash_size);
var hash2 = geomodel.compute_hash(lat, lon); // hash_size = 8 (default)
var box = geomodel.compute_box(hash);
Sample output: > geomodel.compute_hash(47, 29) 'e118e3d5' > geomodel.compute_hash(47, 29, 16) 'e118e3d518e3d518' > geomodel.compute_box("e118e3d5") { northEast: { lat: 47.00225830078125, lon: 29.00390625 }, southWest: { lat: 46.99951171875, lon: 28.9984130859375 }, getNorth: [Function], getSouth: [Function], getWest: [Function], getEast: [Function] } >
For detailed usage see test/index.coffee
Testing
For testing you need to install nodeunit module:
# npm install nodeunit -g
# cd test & nodeunit index.coffee