By default duplicated objects will
be placed on the x-y plane. The center point of all
objects will be on zero height. This is not what you need
for placing objects on top of a terrain surface. To put
objects to the surface of a generated mesh use control
input 'of'. Connect this control input with the data
channel you are already using for the mesh. This will
lift the the duplicated objects on top of your mesh. In
addition you can use the offset slider to move the
objects in z-direction. Use this if your objects looks
like sunken in the mesh.
It is useful to link height scaling
of the mesh with the z-location for object placement. The
simplest way to do this is by introducing a scaling node.
Then scaling can be adjusted by only one parameter in the
scaling node. The height of the mesh and the z-location
follows this parameter.