Model floats above or sinks into the terrain

• The model bottom floats roughly 8 metres after enabling world terrain.
• In a mountainous area the lower half of the model is cut off by the terrain.
• The printed position longitude, latitude and height match the design coordinates exactly.

1. After load, read model.boundingSphere and subtract its centre from the entity position to get the origin offset.
2. Do not sample height right after constructing the Viewer, because terrain tiles have not arrived yet.
3. Use sampleTerrainMostDetailed or wait for globe.tilesLoaded to obtain the real ground height.
4. Check terrainExaggeration, since the same coordinate sits differently under two terrain settings.
5. Check whether heightReference is CLAMP_TO_GROUND and whether the model origin is at its base.

Either set heightReference to Cesium.HeightReference.CLAMP_TO_GROUND on the model, or compute placeHeight as groundHeight minus bottomOffset, for example ground 45 m with a bottom offset of -12.4 m, and place it through Cesium.Transforms.eastNorthUpToFixedFrame. Reset terrainExaggeration to 1.0 while calibrating.

var entity = viewer.entities.add({
  position: Cesium.Cartesian3.fromDegrees(116.391, 39.907, 0),
  model: { uri: '/models/tower.glb' }
});

viewer.scene.globe.tilesLoaded = true;
entity.model.readyPromise.then(function (model) {
  var bs = model.boundingSphere;
  console.log('包围球半径', bs.radius);
  console.log('中心相对原点', Cesium.Cartesian3.subtract(
    bs.center, entity.position.getValue(viewer.clock.currentTime), new Cesium.Cartesian3()
  ));
});

var terrain = await Cesium.createWorldTerrainAsync();
var carto = Cesium.Cartographic.fromDegrees(116.391, 39.907);

var samples = await Cesium.sampleTerrainMostDetailed(terrain, [carto]);
console.log('地面高程', samples[0].height);

• Calling scene.sampleHeight immediately after creating the Viewer, which returns 0 from the ellipsoid rather than the terrain height.
• Baking the correction into the GLB by re-exporting, which breaks as soon as another area has different elevation.