hello, i need to determine the average height of several polysurfaces for a project. is there a command for this in rhino ? thank you very much for your help 
there is no direct vanilla rhino command to do what you are requesting AFAIK.
There are most likely all kinds of scripts floating around out there, GH ways to do this, and of course good old fashioned rhino geometry construction ways to do it.
Anyone who helps you down the line with this will need a file and at least a visual description to help with assisting you on this. Got a screenshot to put here at least?