Render an alpha mask that includes shadows on terrain?

[Lemures]

I have a series of structures set up on geo-located terrain in such a way that the terrain height is relevant. I have matched the SketchUp view with a real-world photo, and want to render the structures and then composite them into the real shot, such that the final shot shows the rendered structures on the real world terrain including the appropriate shadows for the time of day. I think the path forward is to capture the rendered shadows in such a way that I can use them as a layer in photoshop with the appropriate blending mode, such that they "shade" the real-world terrain.

I think I want to find a way to render an alpha mask that does NOT include the ground, such that any geometry that goes into the ground (like a building built into a hill) is not captured by the alpha mask. I also think that I want to find a way to capture the shadows in a render in such a way that they can easily be a separate layer, but I can't figure out how to do this. Is there any way to do either of these?

[Fletch]

I have answered this here:
Subject: How do I best superimpose a rendered image onto a matched photo, keeping shadows?

[Lemures]

Oh, you're right, my apologies! I've been deep into this project and straight up forgot that I had already asked this question. Thank you for your patience and help!