Auto landscape Geometry Node

I am working on a geometry node using curve to define the boundary of the terrain and wanna have the edge of the curve stick to original Z position.

Originally, I tried to find something like "layer weight" in geometry node but in vein. I used curve offset and proximity node to elevate the center region of the curve to form a terrain. However, I dont know how to fix the edge of the curve always in 0 @Z position. 

My questions are :

1) instead of using the way I am doing. Is there any other way to elevate the center of the terrain?

2) How to maintain the edge of the terrain always stick to 0@z position?

* in order to visualize what I am doing, I highlighted the original curve in red pipe and the terrain in green. The edge of the terrain is not touching the red pipe.

Many thanks.