// 相机最低高度
const minimumHeight = 300;
 
// 在渲染阶段前添加事件监听器
viewer.scene.preRender.addEventListener(function () {
 
var eye = viewer.camera.positionCartographic;
 
// 判断相机坐标是否小于阈值，若小于阈值，则保持视点方位，修改相机高度
if (eye.height < minimumHeight) {
    viewer.camera.setView({
        destination: Cesium.Cartesian3.fromRadians(eye.longitude, eye.latitude, minimumHeight),
        orientation: {
        direction: viewer.camera.direction,
        up: viewer.camera.up
    }
    });
}
});