写一段Cesium加载3857地形瓦片的代码

var viewer = new Cesium.Viewer('cesiumContainer');

var terrainProvider = new Cesium.CesiumTerrainProvider({

url : 'https://assets.agi.com/stk-terrain/v1/tilesets/world/tiles',

requestWaterMask : true,

requestVertexNormals : true

});

viewer.terrainProvider = terrainProvider;

viewer.camera.flyTo({

destination : Cesium.Cartesian3.fromDegrees(-122.4194, 37.7749, 2500)

});