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Leaflet

Accessing OS NGD API – Tiles via Leaflet

Leaflet is an open-source JavaScript library for displaying interactive maps on the web or mobile. A simple and lightweight library that will enable you to display and visualise location data and build dynamic applications.

What you'll need

OS NGD API – Tiles added to an API project in the OS Data Hub with an API Key. See for more information.
A text editor like Visual Studio Code or Notepad to edit and save your HTML and JavaScript files.

Set up your HTML file

Create a new HTML file with a text editor (for example, Notepad, Visual Studio Code).
Add the basic HTML structure to your file with a placeholder <div> for the map.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>OS NGD API – Tiles | Template (EPSG:3857) | Leaflet</title>
    
    <!--Add the Ordnance Survey Styling-->
    <link rel="stylesheet" href="https://labs.os.uk/public/os-api-branding/v0.3.1/os-api-branding.css" />
    <script src="https://labs.os.uk/public/os-api-branding/v0.3.1/os-api-branding.js"></script>
    
    <!--Add the Leaflet libraries-->
    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.3/dist/leaflet.css" />
    <link rel="stylesheet" href="https://unpkg.com/maplibre-gl@2.4.0/dist/maplibre-gl.css" />
    <script src="https://unpkg.com/leaflet@1.9.3/dist/leaflet.js"></script>
    <script src="https://unpkg.com/maplibre-gl@2.4.0/dist/maplibre-gl.js"></script>
    <script src="https://unpkg.com/@maplibre/maplibre-gl-leaflet@0.0.19/leaflet-maplibre-gl.js"></script>
    
    <style>
        /* Set the map container size and style */
        body { margin: 0; padding: 0; }
        #map { position: absolute; top: 0; bottom: 0; width: 100%; }
    </style>
</head>
<body>
    
    <!--Create a div element to hold the map-->
    <div id="map"></div>
    
    <!--Add your Javascript code below--> 
    <script>
        // Your Javascript code will go here

    </script>

</body>
</html>

Insert your API Key

To enable access to OS APIs an API Key is required. Inside the <script> tag, add a variable called apiKey, replacing 'INSERT_API_KEY_HERE' with the API Key from your project.
Inside the <script> tag, add another variable called collectionIdwith the collection ID for the OS NGD API – Tiles basemap – ngd-base.

// Set API Key and collection ID
 const apiKey = 'INSERT_API_KEY_HERE';
 
 const collectionId = 'ngd-base';

Adding a fetch and response interceptor

We need to intercept and customise the style request, adding a tiles property to provide a correctly formatted URL and ensure authentication through the apiKey is enabled to make sure that the correct tiles are requested.
Add the following code inside the JavaScript block:

// Modify the JSON style request incorporate a `tiles` property which lists an array of tile endpoints.
// The '&key=' HTTP query parameter is also appended to each tile endpoint to authenticate the request.
// NOTE: The {z}, {x} and {y} template values are replaced with the corresponding integers at runtime.
    const { fetch: originalFetch } = window;
    window.fetch = async (...args) => {
        let [ resource, config ] = args;

        let response = await originalFetch(resource, config);
        if( response.url != `https://api.os.uk/maps/vector/ngd/ota/v1/collections/${collectionId}/styles/3857` )
            return response;

        // Response interceptor.
        const json = () =>
            response.clone().json().then((data) => {
                delete data.sources[ collectionId ].url;
                data.sources[ collectionId ].tiles = [ `https://api.os.uk/maps/vector/ngd/ota/v1/collections/${collectionId}/tiles/3857/{z}/{y}/{x}?key=${apiKey}` ];
                return data;
            });

        response.json = json;
        return response;
    };

Create a map and map view

Initialize the map object using the L.Map class to configure the vector tile layer and the mapOptions variable to define its properties – minZoom, maxZoom, maxBounds, center and zoom.

// Initialize the map.
    const mapOptions = {
        minZoom: 7,
        maxZoom: 20,
        center: [ 50.727589, -3.541809 ],
        zoom: 18,
        maxBounds: [
            [ 49.528423, -10.76418 ],
            [ 61.331151, 1.9134116 ]
        ],
        attributionControl: false
    };

    const map = L.map('map', mapOptions);

    // Load and display vector tile layer on the map.
    const gl = L.maplibreGL({
        style: `https://api.os.uk/maps/vector/ngd/ota/v1/collections/${collectionId}/styles/3857`
    }).addTo(map);

The above code creates the main map instance using the Leaflet library where you can specify various properties:

minZoom and maxZoom: Sets the minimum and maximum zoom level for the map. Users will not be able to go beyond these levels.
maxBounds: Defines the maximum bounds and restricts panning the map.
center: Sets the initial centre point of the map.
zoom: Sets the initial zoom level of the map.

What's next?

Congratulations! You've successfully created a vector map using Leaflet using OS NGD API – Tiles in a few steps.

Now you can continue to explore Ordnance Survey's code examples to learn more about advanced features and functionality, such as adding markers, pop-ups, and additional layers.