Wow, cool work Will.
To adjust the height of a heightfield choose "create heightfield operator" ---> "adjust vertical". Connect the operator
between the heightfield generator and heightfield shader.
This should work, at least it does for me
If you have trouble placing heightfields/objects in front of your camera:
Add same heightfield coordinates as camera coordinates. Then raise your camera a bit and let it face downward. Zoom out a little until you see the boundary box of the heightfield in the render preview. Select the heightfield (double triple clicking whatever...it's a pity sometimes
) and move it to your desired position.
Then press "view camera", NOT "set camera"! to return to your original pov.
Hope this will help.
Martin