Can somebody tell me why I can’t get coordinates inside of ground (NaN), but outside of ground return what it should return?
Here is my playground: Babylon.js Playground
Looks like the ground hasn’t finished being created yet, if you use the onReady callback then you can know when you’re able to start using getHeightAtCoordinates to get its height. 
I’ve deleted my post, because @blake already posted a solution and it was a bit off anyway 