OS Transform

A set of JavaScript helper functions for transforming between the OSGB36/British National Grid (EPSG:27700) and WGS84 (EPSG:4326) coordinate systems; as well as providing support for OS Grid Reference conversions.

Installation

Include the following <script> tag in the header section of your HTML document:

<script src="os-transform.js"></script>

Background

In August 2016, Ordnance Survey released a transformation grid called OSTN15 which improves the accuracy of the transformation between OSGB36/British National Grid and WGS84^[1]. It does this by implementing a 'rubber-sheet' style transformation that works by using bilinear interpolation to essentially apply different transformations in different parts of the country.

You can find more information in relation to the transformation by visiting the OSTN15 for developers page on the Ordnance Survey website; or by reading our guide to coordinate systems in Great Britain (PDF).

Proj4js [requirement]

Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations.

It can be added as a source at the top of your HTML document (before os-transform.js is defined):

<script src="https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.7.0/proj4.js"></script>
<script src="os-transform.js"></script>

Version 2.7.0 of the library includes support for grid based datum adjustments (using the +nadgrids keyword in a coordinate system definition). This means that the OSTN15 can be added to the conversion as follows:

var request = new XMLHttpRequest();
request.onload = function() {
    var arrayBuffer = request.response;
    proj4.nadgrid('OSTN15_NTv2_OSGBtoETRS', arrayBuffer);
    proj4.defs('EPSG:27700', '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs +nadgrids=OSTN15_NTv2_OSGBtoETRS');
    ....
};
request.open('GET', 'OSTN15_NTv2_OSGBtoETRS.gsb');
request.responseType = 'arraybuffer';
request.send();

Although earlier versions of the Proj4js library can also be used, the +nadgrids grid shift won't be supported, i.e. the point coordinate conversion will only be implemented using a simple seven-parameter geodetic transformation:

proj4.defs('EPSG:27700', '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs');

This particular transformation (more commonly known as a Helmert datum transformation) is an averaged transformation between the two datums which will smooth out localised distortions in OSGB36/British National Grid. Because the transformation uses a single set of parameters for the whole country it has a 95% accuracy; and can give errors of up to 3.5m (this value varies across the country).

Usage

os.Transform.toLatLng()

Description: Return latlng from an input easting + northing.

Parameters:

coordinates {object}: The easting + northing to be transformed.
decimals {integer}: [Optional | Default = 7] The specified number of decimal places.

os.Transform.toLatLng({ ea: 337297, no: 503695 });
// Returns { lat: 54.42481, lng: -2.9679374 }

os.Transform.fromLatLng()

Description: Return easting + northing from an input latlng.

Parameters:

coordinates {object}: The latlng to be transformed.
decimals {integer}: [Optional | Default = 2] The specified number of decimal places.

os.Transform.fromLatLng({ lat: 54.42480998276385, lng: -2.96793742245737 });
// Returns { ea: 337297, no: 503695 }

os.Transform.toGridRef()

Description: Return grid reference [plain | encoded | components] from an input easting + northing.

Parameters:

coordinates {object}: The easting + northing to be converted.

os.Transform.toGridRef({ ea: 337297, no: 503695 });
// Returns { text: "NY 37297 03695", html: "NY 37297 03695", letters: "NY", eastings: "37297", northings: "03695" }

os.Transform.fromGridRef()

Description: Return easting + northing from an input grid reference.

Parameters:

gridref {string}: The grid reference to be converted.

os.Transform.fromGridRef("NY 37297 03695");
// Returns { ea: 337297, no: 503695 }

Change Log

Version 0.1.0 (March 2020)

• Initial release.

Version 0.2.0 (June 2020)

• Updated to include _checkBounds() function, which ensures input coordinates are within the permitted bounds (extent of GB).

Version 0.3.0 (February 2021)

• Included support for grid based datum adjustments.
• Minor code refactoring.
• Additional usage examples provided.

Version 0.4.0 (March 2021)

• Added support for 4-figure to 12-figure grid references.
• Updated to include _validateGridRef() function, which validates input grid references.
• toGridRef() function includes components (letters, eastings + northings) of grid reference in returned object.

Notes

^[1] OSTN15 is actually a transformation to European Terrestrial Reference System 1989 (ETRS89), which can be considered as a higher accuracy version of WGS84 for Europe.

Licence

The contents of this repository are licensed under the Open Government Licence 3.0.