So, I talked to ESRI support when I was running 10.1 about this. Just re-ran this in 10.2.
Challenge: extrude volumes between 2 surfaces based on a polygon extent.
Result: Multi-patches come out unclosed, fortunately now the close tool functions as expected and actually does close most of them. Unfortunately the resulting multi-patches appear to be created to extend from the highest point on the top surface to the lowest point on the bottom surface (within the polygon extent) at least some of the time...[ATTACH=CONFIG]27000[/ATTACH][ATTACH=CONFIG]27001[/ATTACH]
In the attached images the partially transparent green blocks are the extruded multi-patches and the grey TIN is the bottom of the two original TIN's (the top is nearly flat)
You can see it illustrated even more clearly in the third image, where the multi-patch nodes etc. are highlighted by a selection.
[ATTACH=CONFIG]27002[/ATTACH]
I'll contact tech support again, and post any updates, but I figured I'd throw this problem on the forums too, just in case anyone was relying on these volume calcs.
