I recommend to watch this at first as Jeff Lait explains the basic principles of heightfields.
The first part of this video shows a basic workflow of creating terrains in Houdini.
Ari Danesh talks about the new features of H17 from 54:18
MapBox: from 51:26
I recommend installing the Game Tools for Houdini, as it has tools that can be useful for terrain work. Maybe the most interesting is the MapBox node which allows us to directly browse the Earth inside Houdini, then we can select a specific area and download the elevation model and the satellite phototexture of it. It has a heightfield output option, so the entire planet is at our hands.