How does Base Terrain work internally in Cesium ion?

Cesium ion
1

Hi, I am trying to understand how “Base Terrain” works internally in Cesium ion.

When uploading terrain data, Cesium ion allows selecting a Base Terrain, and the documentation says that the two are “combined to create a new global terrain layer”.

However, I am confused about what “combined” actually means in practice.

Here are the specific things I want to understand:

  1. Is the terrain data physically merged during tiling, or is it more like a reference/fallback system?
  2. If my uploaded terrain only covers a small region, does Cesium use my terrain where available and fall back to the Base Terrain elsewhere?
  3. When rendering in CesiumJS, only one terrainProvider is used. How are these two terrains handled internally?
  4. Why does the total asset size not significantly increase even when a Base Terrain is selected?
  5. Is this behavior defined somewhere in the official documentation (e.g., how tile requests are resolved between the two terrains)?

From my observation, it seems like:

  • Tiles from my terrain are used where available
  • Missing tiles are filled from the Base Terrain

But I want to confirm if this understanding is correct, and how it is implemented internally.

Thanks!