Long lat coordinates on primitive object

Hi all,

I have a 3D model of a city and surrounding area, and need to clip some irregular shape out of it, which will be replaced by new buildings. The problem is that the longitude and latitude coordinates used for clipping do not match between the globe and the 3D model. Can someone help me please?

The model is a 3D landscape and buildings of a Dutch municipality named Alkmaar ( lat: 52.629435, Lon: 4.7536084). This includes the city and a large rural area in the South-West. The “boundingSphere.center” of this complete object is in the rural area at Lat: 52.600843, Lon: 4.817590. The data is in a B3DM file stored on a private storage, which I am not allowed to share (yet), so the model won’t load in the sandcastle example provided below. To load the model I only refer to the tileset.json and the tiles load at the correct position on the globe.

I want define the clipping area with the actual long lat coordinates on the globe. The clipping plane is however oriented relative to the boundingSphere.center of the 3D model and not to the world coordinates. If I do use actual lon lat coordinates, the clipping plane is not visible (it is probably located somewhere far away).

I played around with the coordinates of the clipping plane until they were clipping somewhere on the 3D Model. This happen to around Afrika, using longitude 0, latutude -45. The images below show the effect of the same clipping coordinates on both the globe and the model.

For completeness, here is the model zoomed out. The red line is the border of the 3D model, the blue line is the clipped area.

What do I need to do so that clipping at:
“new Cesium.Cartesian3.fromDegrees( 52.6008, 4.7536, 0)” actually clips on that location on my 3D model.

Thanks a lot for your help, much appreciated!

Sandcastle example code (sandcastle code

Based on example of St Helens: st helens demo