all. I am new to Blender. I have modeled a terrain by joining the edges of different contour lines which I imported from a DXF file. However, I have no idea how to continue creating a small hill on top of a closed polygon or surface. Next, I would like also to know whether there is any tutorial that I could refer to in order to know how to place a road nicely on the terrain surface. Please find the Blender file if needed. Thank you.
For your small hill, I'd use Loop tools. It's an add-on that comes with your standard Blender.
Select two layers of vertices and Bridge them.
It's wise to subdivide longer edges since Looptools Bridge will not handle them great in this situation.
As for your road, you could use a bunch of modifiers for that. Mainly Shrinkwrap, and a few others that increase the detail of the mesh geometry (more vertices) to help Shrinkwrap work better (give it more vertices to work with).
And finally apply Shrinkwrap (and some other modifiers to make it look a little bit nicer) The road is now projected on to the mountain surface from a top down perspective. (Note how the "Mode:" is set to Project.)
The road is shaped a bit funny where there is no mountain underneath it; that might need some manual editing/tweaking I'm afraid.