CityGML buildings not clamping to ground properly

Hey again community,

I have a question regarding cityGML buildings positioned wrong in the z-axis. I have created cityGML buildings, they are in LOD 2. When I upload them to Ion the are not clamped to terrain properly. I’m unsure if this problem is in cesium or if something gone wrong in the creation process via FME. When they are in kmz format they work properly and are clamped correct. But we need to work with cityGML. I have checked the terrains height in cesiumJS and it is correct.

I take one buildings location as example:
The terrain is 132 meters above sea lv.
But when I offset the building with min z they still float in the air, about 28 meters in the air.
The interesting thing is, that places the building in 160 meters above the sea lv, which I remember the terrain was before when I didn’t fuse it properly (did not assign ellipsoid on upload).
Another thing is that when I inspect the building via fme I can’t seem to find a single coordinate z value that is 160 meters above sea lv. The infobox of each building also displays TerrainHeight, which should be the height the building should be placed on.


To add: I’m aware that building can’t be clamped to ground but still it should not be floating

If the buildings is not offset with min z they float even higher. But when I inspect with an underlaying pointcloud in fme. The building sits perfect in z axis (no offset).

Does anyone have an idea why the buildings float in cesium? It would make sense if they float n amout of meters in the air corresponding to the zmin but it’s higher than that… The z heights is not constant either each building differs just a bit.

Thanks, Albin