Use terrain data when calculating z-values

[laurenwalker]

Currently we set a z-value on each point in each feature that is set at a flat height above the Earth's ground.

However, when the terrain is turned on in Cesium, these features may disappear under the terrain since they do not drape over the terrain like raster imagery does.

I've researched this and it appears that Cesium does not have a handy clamp-to-ground feature during the rendering in Cesium (and if they did, it would likely be slow). The source data needs to have the terrain data baked in.

From Cesium's Community Board:

How to get added tileset always above the terrain?

https://community.cesium.com/t/how-to-get-added-tileset-always-above-the-terrain/18506/4

I see two options:

1. Sample the corners and center of your terrain, and map it to a bounding area, and if the 3DTiles are below the terrain you can lift it through a Transform.

2. Use transparent globe/terrain with a distance factor, so from a distance the terrain is solid, but when you move closer to the model / pointcloud it becomes more transparent to show the parts normally hidden.

3D Tiles Data Clamp to the Terrain

https://community.cesium.com/t/3d-tiles-data-clamp-to-the-terrain/15099

Yes, the clamp-to-terrain step [in Cesium Ion] operates on the source [..] data and can’t be performed retroactively on a tileset.