cesium 禁止相機進入地下

cesium 禁止相機進入地下
//設置相機禁止進入地下

//能解決的請回復一下
viewer.clock.onTick.addEventListener(function () {
    // 獲取高度
    var extent = Math.ceil(viewer.camera.positionCartographic.height);
    if(viewer.camera.positionCartographic.height<100){
        var position = viewer.camera.position;
        position.z = position.z + 100;
        var heading = viewer.camera.heading;
        var roll = viewer.camera.roll;
        console.log("pos:" + position + " head:" + heading + " roll:" + roll + " pitch:" + viewer.camera.pitch);
        viewer.camera.setView({
            destination: position,
            orientation: {
                heading: heading,
                pitch: viewer.camera.pitch,
                roll: roll
            }
        });
    }
    if (viewer.camera.pitch > Cesium.Math.toRadians(-3)) {
        var position = viewer.camera.position;
        position.z = position.z + 1000;
        var heading = viewer.camera.heading;
        var roll = viewer.camera.roll;
        console.log("pos:" + position + " head:" + heading + " roll:" + roll + " pitch:" + viewer.camera.pitch);
        viewer.camera.setView({
            destination: position,
            orientation: {
                heading: heading,
                pitch: Cesium.Math.toRadians(-10),
                roll: roll
            }
        });
    }
});