Hi @AOCG,
This is possible by inverting the clipping effect. Here’s a guide for how to do this: How to use multiple terrain? - #5 by agallegos
Make sure to uncheck the “exclude tiles inside” parameter on the Polygon Raster Overlay component.
Be aware that this is a material-only effect, and so the tiles outside the selection will still be loading invisibly and can be collided with. Eventually, we’d like to improve this system. You can track the status of this feature here: Add a parameter for inverting Cartographic Polygon · Issue #721 · CesiumGS/cesium-unreal · GitHub
-Alex