Javascript设备指南针数据总是从~90度开始

Question

我使用here和here中的代码来确定通过相机查看手机的方向。

初始化后，代码看起来非常流畅和一致。问题是，当我加载代码时，无论我的设备指向哪个方向，它都变为90度（实际为89.7）。这是一个显而易见的问题，因为现在我不能简单地说dir < 45 && dir > 315 = "south"。

这是由于我的特定硬件/软件吗？ （Pixel XL，Chrome移动版）或我在代码中做错了什么？

更新：我遇到与this one from ArcGIS和this Marine Compass demo等复杂指南针相同的行为。

以下是完整代码：

function compassHeading(alpha, beta, gamma) {

    // Convert degrees to radians
    var alphaRad = alpha * (Math.PI / 180);
    var betaRad = beta * (Math.PI / 180);
    var gammaRad = gamma * (Math.PI / 180);

    // Calculate equation components
    var cA = Math.cos(alphaRad);
    var sA = Math.sin(alphaRad);
    var cB = Math.cos(betaRad);
    var sB = Math.sin(betaRad);
    var cG = Math.cos(gammaRad);
    var sG = Math.sin(gammaRad);

    // Calculate A, B, C rotation components
    var rA = - cA * sG - sA * sB * cG;
    var rB = - sA * sG + cA * sB * cG;
    var rC = - cB * cG;

    // Calculate compass heading
    var compassHeading = Math.atan(rA / rB);

    // Convert from half unit circle to whole unit circle
    if(rB < 0) {
        compassHeading += Math.PI;
    }else if(rA < 0) {
        compassHeading += 2 * Math.PI;
    }

    // Convert radians to degrees
    compassHeading *= 180 / Math.PI;

    return compassHeading;

}


document.addEventListener("DOMContentLoaded", function(event) {

    if (window.DeviceOrientationEvent) {
        window.addEventListener('deviceorientation', function(eventData) {
            let tiltLR = eventData.gamma;
            let tiltFB = eventData.beta;
            let dir = eventData.alpha;

            deviceOrientationHandler(tiltLR, tiltFB, dir);
        }, false);
    }

    function deviceOrientationHandler(tiltLR, tiltFB, dir) {
        document.getElementById("tiltLR").innerHTML = 'tiltLR: ' + Math.ceil(tiltLR);
        document.getElementById("tiltFB").innerHTML = 'tiltFB: ' + Math.ceil(tiltFB);
        let heading = compassHeading(dir, tiltFB, tiltLR);
        if (heading >= 0 && heading < 90) {
            document.getElementById("direction").innerHTML = 'direction: S (' + heading + ')';
        } else if (heading >= 90 && heading < 180) {
            document.getElementById("direction").innerHTML = 'direction: W (' + heading + ')';
        } else if (heading >= 180 && heading < 270) {
            document.getElementById("direction").innerHTML = 'direction: N (' + heading + ')';
        } else if (heading >= 270 && heading <= 360) {
            document.getElementById("direction").innerHTML = 'direction: E (' + heading + ')';
        } else {
            document.getElementById("direction").innerHTML = 'direction: ? (' + heading + ')';
        }

    }

});