Intersection Points with Topography

hey

i've got a topography and a layer with outlines of buildings floating over it.

i calculate the midpoint of my buildings, draw a line from this point to the surface underneath that point. and i want to extrude the same geometryof the building on the intersection of the surface. 

my problem is that if i select curves that intersect in different layers of my topography, they all get dragged to a point near it but on another surface.

i know that the problem is that the midpoint of the geometry gets the closest point from the collection of all the surface-intersection points. 

how can i just select the intersection-points for the point above?

cheers matt