# geomodeljs

An implementation of the Geomodel/Geocell concept.

## Usage no npm install needed!

``````<script type="module">
import geomodeljs from 'https://cdn.skypack.dev/geomodeljs';
</script>``````

# 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
``````