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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Query terrain elevation along a line</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no">
<link href="https://api.mapbox.com/mapbox-gl-js/v3.4.0/mapbox-gl.css" rel="stylesheet">
<script src="https://api.mapbox.com/mapbox-gl-js/v3.4.0/mapbox-gl.js"></script>
<style>
body { margin: 0; padding: 0; }
#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<style>
    #chart-container {
        height: 25%;
        width: 100%;
        position: absolute;
        top: 0;
        left: 0;
        right: 0;
        z-index: 10;
        background-color: rgba(255, 255, 255, 0.9);
        text-align: center;
    }

    #chart-inner-container {
        position: relative;
        margin: auto;
        height: 100%;
        width: 100vw;
    }
</style>

<script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
<script src="https://unpkg.com/@turf/turf@6/turf.min.js"></script>
<div id="map">
    <div id="chart-container">
        <div id="chart-inner-container">
            <canvas id="chart-canvas"></canvas>
        </div>
    </div>
</div>

<script>
	// TO MAKE THE MAP APPEAR YOU MUST
	// ADD YOUR ACCESS TOKEN FROM
	// https://account.mapbox.com
	mapboxgl.accessToken = 'pk.eyJ1IjoiemVuc2FuIiwiYSI6ImNsandiMWx4djFsd3MzY3RodG1vNm55bm4ifQ.DpwvbO12Bli7Cbok5mMU7Q';
    (async () => {
        const map = new mapboxgl.Map({
            container: 'map', // container ID
            style: 'mapbox://styles/mapbox/outdoors-v12', // style URL
            bounds: [ 113.94742745, 22.53259842, 113.94742745, 22.53259842]
        });

        // default start and end point are defined as a geoJSON LineString
        const lineData = {
            type: 'Feature',
            geometry: {
                type: 'LineString',
                coordinates: [
                    [ 113.94742745, 22.53259842],
                    [ 113.94742745, 22.53259842]
                ]
            },
            properties: {}
        };

        map.on('style.load', () => {
            // add source and layer for rendering the line
            map.addSource('line-data', {
                type: 'geojson',
                data: lineData
            });

            map.addLayer({
                id: 'line-line-data',
                type: 'line',
                source: 'line-data',
                paint: {
                    'line-width': 4,
                    'line-color': '#37a2eb'
                }
            });

            // add the digital elevation model tiles
            map.addSource('mapbox-dem', {
                type: 'raster-dem',
                url: 'mapbox://mapbox.mapbox-terrain-dem-v1',
                tileSize: 512,
                maxzoom: 20
            });
            map.setTerrain({ 'source': 'mapbox-dem', 'exaggeration': 1 });

            // add draggable markers
            const marker0 = new mapboxgl.Marker({
                draggable: true,
                color: '#83f7a0'
            })
                .setLngLat(lineData.geometry.coordinates[0])
                .addTo(map);

            const marker1 = new mapboxgl.Marker({
                draggable: true,
                color: '#ed6461'
            })
                .setLngLat(lineData.geometry.coordinates[1])
                .addTo(map);

            // as the user drags a marker, update the data for the line and re-render it with setData()
            const updateLineData = (e, position) => {
                const { lng, lat } = e.target.getLngLat();
                lineData.geometry.coordinates[position] = [lng, lat];
                map.getSource('line-data').setData(lineData);
            };

            marker0.on('drag', (e) => {
                updateLineData(e, 0);
            });
            marker0.on('dragend', updateElevationProfile);

            marker1.on('drag', (e) => {
                updateLineData(e, 1);
            });
            marker1.on('dragend', updateElevationProfile);
        });

        const myLineChart = new Chart(document.getElementById('chart-canvas'), {
            type: 'line',
            data: {
                labels: [],
                datasets: []
            },
            options: {
                plugins: {
                    legend: {
                        display: false
                    },
                    title: {
                        display: true,
                        align: 'start',
                        text: 'Elevation (m)'
                    }
                },
                maintainAspectRatio: false,
                responsive: true,
                scales: {
                    x: {
                        grid: {
                            display: false
                        }
                    },
                    y: {
                        min: -10,
                        grid: {
                            display: false
                        }
                    }
                },
                elements: {
                    point: {
                        radius: 0
                    }
                },
                layout: {
                    padding: {
                        top: 6,
                        right: 20,
                        bottom: -10,
                        left: 20
                    }
                }
            }
        });

        function updateElevationProfile() {
            // split the line into 1km segments
            const chunks = turf.lineChunk(lineData, 1).features;

            // get the elevation for the leading coordinate of each segment
            const elevations = [
                ...chunks.map((feature) => {
                    return map.queryTerrainElevation(
                        feature.geometry.coordinates[0]
                    );
                }),
                // do not forget the last coordinate
                map.queryTerrainElevation(
                    chunks[chunks.length - 1].geometry.coordinates[1]
                )
            ];

            // add dummy labels
            myLineChart.data.labels = elevations.map(() => '');
            myLineChart.data.datasets[0] = {
                data: elevations,
                fill: false,
                tension: 0.4
            };
            myLineChart.update();
        }

        // trigger the first chart draw after the DEM tiles have loaded
        await map.once('idle');
        updateElevationProfile();
    })();
</script>

</body>
</html>