I am new to autocad, I am running 2009. I use a third party duct drawing progam that runs thru Autocad 2009. Now my question is how do I set up the drawing to be at zero elevation..or for that matter the elevation that the xrefs come in.(I use arch. drawings as the background).

the next question is how do I set up the 2nd floor elevation...which I assume would be like setting up the first floor elevation.
Thanks a bunch in advance...this site has help me a ton so far!
James