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Leaflet

Accessing OS NGD API – Features 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 Maps API and OS NGD API – Features added to an API project in the OS Data Hub with an API Key.
A text editor like Visual Studio Code or Notepad to edit and save your HTML and JavaScript files.

Create a basic map

Step 1: 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 – Features | Template (EPSG:3857) | Leaflet</title>
    
    <!--Add the Ordnance Survey Styling-->
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/OrdnanceSurvey/os-api-branding@0.3.1/os-api-branding.css" />
    <script src="https://cdn.jsdelivr.net/gh/OrdnanceSurvey/os-api-branding@0.3.1/os-api-branding.js"></script>
    
    <!--Add the Leaflet libraries-->
    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" />
    <script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.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>

Step 2: Insert your API Key and OS NGD collection

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.
Add a variable called collectionID, replacing 'INSERT_COLLECTIONID_HERE' with the collection ID for the desired OS NGD feature type and version (for example, bld-fts-buildingpart-1).

// Set API Key 
 const apiKey = 'INSERT_API_KEY_HERE';
 
 const collectionId= 'INSERT_COLLECTIONID_HERE';

Step 3: Add a basemap

Define the configuration options for the map, defining minZoom, maxZoom, center, zoom, maxBounds, attributionControl.
- minZoom and maxZoom: Sets the minimum and maximum zoom level for the map. Users will not be able to go beyond these levels.
- center: Sets the initial centre point of the map.
- zoom: Sets the initial zoom level of the map.
- maxBounds: Defines the maximum bounds and restricts panning the map.
- style: Defines the style of the map, configured via a URL pointing at the style specified.
- attributionControl: When set to 'false', it hides the attribution control which displays map credits.
Initialize the map with the id of the <div> element and the configuration option defined in mapOptions.
Using the 'L.tileLayer' method, specify the basemap layer for OS Maps API, which includes your API Key to load the tiles to your map.

// 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 ZXY tile layer on the map.
    const basemap = L.tileLayer(`https://api.os.uk/maps/raster/v1/zxy/Light_3857/{z}/{x}/{y}.png?key=${apiKey}`, {
        maxZoom: 20
    }).addTo(map);

Step 4: Add an OS NGD API – Features layer

Create a function called fetchFeatures that fetches the API based on the current map extent (bounding box) by generating a bbox string.
Construct the API request URL to fetch OS NGD data from OS NGD API – Features. The URL includes the collectionId, bbox and apiKey.
Once the features have been returned in JSON, update the source data of the map's layers to display the features.

// Add layer group to make it easier to add or remove layers from the map.
    const lyrGroup = new L.layerGroup().addTo(map);

// Define an asynchronous function to fetch and display the NGD Features API features.
    async function fetchFeatures(bounds) {
        // Generate a BBOX string for the map extent.
        const bbox = bounds.toBBoxString();

        // Construct the NGD Features API request URL.
        const url = `https://api.os.uk/features/ngd/ofa/v1/collections/${collectionId}/items?key=${apiKey}&bbox=${bbox}`;

        // Fetch features from the API endpoint.
        const features = await fetch(url).then(response => response.json());

        // Parse the GeoJSON data and display it on the map.
        lyrGroup.clearLayers().addLayer(L.geoJSON(features));
    }

// Get the visible map bounds (BBOX).
    let bounds = map.getBounds();

// Initial fetch and display of features.
    fetchFeatures(bounds);

Step 5: Load and update features on the map dynamically

The map.on('moveend',...) event handler fetches and updates the features based on the map's current extent.

// Add event which will be triggered when the map has finshed moving (pan + zoom).
// Implements a simple strategy to only request data when the map viewport invalidates
// certain bounds.
    map.on('moveend', function() {
        let bounds1 = new L.latLngBounds(bounds.getSouthWest(), bounds.getNorthEast()),
            bounds2 = map.getBounds();

        if( JSON.stringify(bounds) !== JSON.stringify(bounds1.extend(bounds2)) ) {
            bounds = bounds2;
            fetchFeatures(bounds);
        }
    });

Features within the viewport extent will load initially (first 100 features) and will continue to load as you pan and zoom across the map.

What's next?

Congratulations! You've successfully created a map using Leaflet and added an OS NGD layer using OS NGD API – Features 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.