gbify-geojson

Reproject GeoJSON between OSGB36 GB National Grid and WGS84 CRSs.

Description

The default reference system used by GeoJSON to describe the geospatial data contained is usually ESPG:4326, cartesian coordinates are referenced using the WGS84 datum.

With this project we can reproject to or from the coordinate reference system WGS84 and the projected local coordinate system OSGB36, EPSG:27700, British National Grid. The library will reproject all coordinate data in a GeoJSON object to / from these reference systems.

This library uses proj4js to do the tranformation.

An example GeoJSON reprojected;

// WGS84
{
  "type": "Feature",
  "properties": {
    "popupContent": "Golden Square, Soho, London."
  },
  "geometry": {
    "type": "Point",
    "coordinates": [
      -0.1371467113494873,
      51.511566985991124
    ]
  }
}
 
// OSGB36
{
  "type": "Feature",
  "properties": {
    "popupContent": "Golden Square, Soho, London."
  },
  "geometry": {
    "type": "Point",
    "coordinates": [
      529368.34,
      180826.93
    ]
  }
}

Caveats

In using this project, please note the following points - these may be fixed in future versions.

• We ignore the CRS property specified by GeoJSON spec. This has no impact on tranformation and is removed if present.
• A bounding box member is reprojected, this probably is not correct.

Web example

This library is used by gb-geojson project that allows you to view, create, edit, transform GeoJSON map data in British National Grid reference system and WGS84. The app reprojects between each CRS when updated, you can try it out http://rob-murray.github.io/gb-geojson/.

Getting started

You can get hold of the code with npm and it should work fine with browserify. Or you can just manually import the source.

Dependency management

With npm:

$ npm install gbify-geojson

Interface

Given the library is loaded

> gbify = require('gbify-geojson')

toOSGB36(geoJson)

Reproject the geoJson object to EPSG:27700.

> var point = {"type":"Feature","properties":{"popupContent":"Golden Square, Soho, London."},"geometry":{"type":"Point","coordinates":[-0.1371467113494873, 51.511566985991124]}}
> gbify.toOSGB36(point)
{ type: 'Feature',
  properties: { popupContent: 'Golden Square, Soho, London.' },
  geometry:
   { type: 'Point',
     coordinates: [ 529368.34, 180826.93 ] } }

toWGS84(geoJson)

Reproject the geoJson object to EPSG:4326.

> var poly = {"type":"Feature","properties":{"popupContent":"I am SU43."},"geometry":{"type":"Polygon","coordinates":[[[440000,130000],[450000,130000],[450000,140000],[440000,140000],[440000,130000]]]}}
> gbify.toWGS84(poly) // JSON.stringify(gbify.toWGS84(poly))
'{"type":"Feature","properties":{"popupContent":"I am SU43."},"geometry":{"type":"Polygon","coordinates":[[[-1.43051,51.067945],[-1.287804,51.067163],[-1.286421,51.157077],[-1.429404,51.157861],[-1.43051,51.067945]]]}}'

Development

Run tests

Run the tests with mocha and expect.js.

$ npm test

Alternatives

The excellent reproject library was a basis for building this specific OS GB flavour.

GDAL can do this also with something like this to reproject OSGB36 > WGS84.

$ ogr2ogr -s_srs EPSG:27700 -f "GeoJSON" output.json input.json -t_srs EPSG:4326

Contributions

Please use the GitHub pull-request mechanism to submit contributions.

License

This project is available for use under the MIT software license. See LICENSE