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Fly to a location based on scroll position

Scroll down through the story and the map will fly to the chapter's location.

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<html lang="en">
<head>
    <title>Fly to a location based on scroll position</title>
    <meta property="og:description" content="Scroll down through the story and the map will fly to the chapter's location." />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/maplibre-gl@4.4.0/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/maplibre-gl@4.4.0/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<style>
    #map {
        position: fixed;
        width: 50%;
    }
    #features {
        width: 50%;
        margin-left: 50%;
        font-family: sans-serif;
        overflow-y: scroll;
        background-color: #fafafa;
    }
    section {
        padding: 25px 50px;
        line-height: 25px;
        border-bottom: 1px solid #ddd;
        opacity: 0.25;
        font-size: 13px;
    }
    section.active {
        opacity: 1;
    }
    section:last-child {
        border-bottom: none;
        margin-bottom: 200px;
    }
</style>

<div id="map"></div>
<div id="features">
    <section id="baker" class="active">
        <h3>221b Baker St.</h3>
        <p>
            November 1895. London is shrouded in fog and Sherlock Holmes and
            Watson pass time restlessly awaiting a new case. "The London
            criminal is certainly a dull fellow," Sherlock bemoans. "There have
            been numerous petty thefts," Watson offers in response. Just then a
            telegram arrives from Sherlock's brother Mycroft with a mysterious
            case.
        </p>
    </section>
    <section id="aldgate">
        <h3>Aldgate Station</h3>
        <p>
            Arthur Cadogan West was found dead, head crushed in on train tracks
            at Aldgate Station at 6AM Tuesday morning. West worked at Woolwich
            Arsenal on the Bruce-Partington submarine, a secret military
            project. Plans for the submarine had been stolen and seven of the
            ten missing papers were found in West's possession. Mycroft implores
            Sherlock to take the case and recover the three missing papers.
        </p>
    </section>
    <section id="london-bridge">
        <h3>London Bridge</h3>
        <p>
            Holmes and Watson's investigations take them across London. Sherlock
            deduces that West was murdered elsewhere, then moved to Aldgate
            Station to create the illusion that he was crushed on the tracks by
            a train. On their way to Woolwich Sherlock dispatches a telegram to
            Mycroft at London Bridge: "Send list of all foreign spies known to
            be in England, with full address."
        </p>
    </section>
    <section id="woolwich">
        <h3>Woolwich Arsenal</h3>
        <p>
            While investigating at Woolwich Arsenal Sherlock learns that West
            did not have the three keys&mdash;door, office, and
            safe&mdash;necessary to steal the papers. The train station clerk
            mentions seeing an agitated West boarding the 8:15 train to London
            Bridge. Sherlock suspects West of following someone who had access
            to the Woolwich chief's keyring with all three keys.
        </p>
    </section>
    <section id="gloucester">
        <h3>Gloucester Road</h3>
        <p>
            Mycroft responds to Sherlock's telegram and mentions several spies.
            Hugo Oberstein of 13 Caulfield Gardens catches Sherlock's eye. He
            heads to the nearby Gloucester Road station to investigate and
            learns that the windows of Caulfield Gardens open over rail tracks
            where trains stop frequently.
        </p>
    </section>
    <section id="caulfield-gardens">
        <h3>13 Caulfield Gardens</h3>
        <p>
            Holmes deduces that the murderer placed West atop a stopped train at
            Caulfield Gardens. The train traveled to Aldgate Station before
            West's body finally toppled off. Backtracking to the criminal's
            apartment, Holmes finds a series of classified ads from
            <em>The Daily Telegraph</em> stashed away. All are under the name
            Pierrot: "Monday night after nine. Two taps. Only ourselves. Do not
            be so suspicious. Payment in hard cash when goods delivered."
        </p>
    </section>
    <section id="telegraph">
        <h3>The Daily Telegraph</h3>
        <p>
            Holmes and Watson head to The Daily Telegraph and place an ad to
            draw out the criminal. It reads: "To-night. Same hour. Same place.
            Two taps. Most vitally important. Your own safety at stake.
            Pierrot." The trap works and Holmes catches the criminal: Colonel
            Valentine Walter, the brother of Woolwich Arsenal's chief. He
            confesses to working for Hugo Oberstein to obtain the submarine
            plans in order to pay off his debts.
        </p>
    </section>
    <section id="charing-cross">
        <h3>Charing Cross Hotel</h3>
        <p>
            Walter writes to Oberstein and convinces him to meet in the smoking
            room of the Charing Cross Hotel where he promises additional plans
            for the submarine in exchange for money. The plan works and Holmes
            and Watson catch both criminals.
        </p>
        <small id="citation">
            Adapted from
            <a href="https://www.gutenberg.org/files/2346/2346-h/2346-h.htm"
                >Project Gutenberg</a
            >
        </small>
    </section>
</div>
<script>
    const map = new maplibregl.Map({
        container: 'map',
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL',
        center: [-0.15591514, 51.51830379],
        zoom: 15.5,
        bearing: 27,
        pitch: 45
    });

    const chapters = {
        'baker': {
            bearing: 27,
            center: [-0.15591514, 51.51830379],
            zoom: 15.5,
            pitch: 20
        },
        'aldgate': {
            duration: 6000,
            center: [-0.07571203, 51.51424049],
            bearing: 150,
            zoom: 15,
            pitch: 0
        },
        'london-bridge': {
            bearing: 90,
            center: [-0.08533793, 51.50438536],
            zoom: 13,
            speed: 0.6,
            pitch: 40
        },
        'woolwich': {
            bearing: 90,
            center: [0.05991101, 51.48752939],
            zoom: 12.3
        },
        'gloucester': {
            bearing: 45,
            center: [-0.18335806, 51.49439521],
            zoom: 15.3,
            pitch: 20,
            speed: 0.5
        },
        'caulfield-gardens': {
            bearing: 180,
            center: [-0.19684993, 51.5033856],
            zoom: 12.3
        },
        'telegraph': {
            bearing: 90,
            center: [-0.10669358, 51.51433123],
            zoom: 17.3,
            pitch: 40
        },
        'charing-cross': {
            bearing: 90,
            center: [-0.12416858, 51.50779757],
            zoom: 14.3,
            pitch: 20
        }
    };

    // On every scroll event, check which element is on screen
    window.onscroll = function () {
        const chapterNames = Object.keys(chapters);
        for (let i = 0; i < chapterNames.length; i++) {
            const chapterName = chapterNames[i];
            if (isElementOnScreen(chapterName)) {
                setActiveChapter(chapterName);
                break;
            }
        }
    };

    let activeChapterName = 'baker';
    function setActiveChapter(chapterName) {
        if (chapterName === activeChapterName) return;

        map.flyTo(chapters[chapterName]);

        document.getElementById(chapterName).setAttribute('class', 'active');
        document.getElementById(activeChapterName).setAttribute('class', '');

        activeChapterName = chapterName;
    }

    function isElementOnScreen(id) {
        const element = document.getElementById(id);
        const bounds = element.getBoundingClientRect();
        return bounds.top < window.innerHeight && bounds.bottom > 0;
    }
</script>
</body>
</html>