Bimbam Often when I add a 3D ground plane I subconsciously start thinking of 2D positional co-ordinates (X/Y) again, but guess that's just a quirk of the brain trying to abstract projections of 3D on a 2D screen.
I think this sort of makes sense, for historical reasons. 2D to me seems like X / Y, and old maps were flat (before they knew the Earth was round, so ignore curvature). Naturally, then, Z would be up. However, if you draw a drawing, from the side (like old cave paintings or from Egypt) then X would be horizontal and Y would be vertical, making Y up. Or if you look at CRT TVs, X and Y are naturally horizontal and vertical, and 2D games came first,so when 3D games arrived, it makes sense that Z is going into the screen (or out of the screen). Then there are conventions in different fields, like math, engineering, etc. that don't agree. So with all this history, there really is no correct answer.