[gameprogrammer] selection / picking on terrain geometry

I must be working on a project because, thats right folks, I have another question :)
:)

Assuming standard 3D RTS with 3D terrain and a 3rd person 45 degree camera (fixed or not, I havent decided yet)

Using OpenGL selection / picking it seems that selection of units in an RTS game, using a mouse, is relatively easy.
However I am having trouble figuring out the best way to click on a point on the screen where there are no units, and have the game know the point on the terrain, where the user clicked.
The terrain is 1 large piece of geometry that is generated from heightmap and optimised. I cannot just give it a name as clicking anywhere will just return the selection stack with the name of the terrain.
How can I find the point on the terrain where I was directing my click.

An example might be selecting some units and then clicking on an empty space to "move here"

Any help or suggestions, including further reading links to books or sites that tackle the subject, is appreciated.
Thankyou

Josh Stewart


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