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Show drawn polygon area

Use mapbox-gl-draw to draw a polygon and Turf.js to calculate its area in square meters.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Show drawn polygon area</title>
    <meta property="og:description" content="Use mapbox-gl-draw to draw a polygon and Turf.js to calculate its area in square meters." />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/[email protected]/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/[email protected]/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<style>
    .calculation-box {
        height: 75px;
        width: 150px;
        position: absolute;
        bottom: 40px;
        left: 10px;
        background-color: rgba(255, 255, 255, 0.9);
        padding: 15px;
        text-align: center;
    }

    p {
        font-family: 'Open Sans';
        margin: 0;
        font-size: 13px;
    }
</style>

<script src="https://www.unpkg.com/[email protected]/turf.min.js"></script>
<script src="https://www.unpkg.com/@mapbox/[email protected]/dist/mapbox-gl-draw.js"></script>
<link
    rel="stylesheet"
    href="https://www.unpkg.com/@mapbox/[email protected]/dist/mapbox-gl-draw.css"
    type="text/css"
/>
<div id="map"></div>
<div class="calculation-box">
    <p>Draw a polygon using the draw tools.</p>
    <div id="calculated-area"></div>
</div>

<script>
    MapboxDraw.constants.classes.CONTROL_BASE  = 'maplibregl-ctrl';
    MapboxDraw.constants.classes.CONTROL_PREFIX = 'maplibregl-ctrl-';
    MapboxDraw.constants.classes.CONTROL_GROUP = 'maplibregl-ctrl-group';

    const map = new maplibregl.Map({
        container: 'map', // container id
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL', //hosted style id
        center: [-91.874, 42.76], // starting position
        zoom: 12 // starting zoom
    });

    const draw = new MapboxDraw({
        displayControlsDefault: false,
        controls: {
            polygon: true,
            trash: true
        }
    });
    map.addControl(draw);

    map.on('draw.create', updateArea);
    map.on('draw.delete', updateArea);
    map.on('draw.update', updateArea);

    function updateArea(e) {
        const data = draw.getAll();
        const answer = document.getElementById('calculated-area');
        if (data.features.length > 0) {
            const area = turf.area(data);
            // restrict to area to 2 decimal points
            const roundedArea = Math.round(area * 100) / 100;
            answer.innerHTML =
                `<p><strong>${
                    roundedArea
                }</strong></p><p>square meters</p>`;
        } else {
            answer.innerHTML = '';
            if (e.type !== 'draw.delete')
                alert('Use the draw tools to draw a polygon!');
        }
    }
</script>
</body>
</html>