Area measured by level

[tom]

Hello,
Since a while I'm trying to get some lines in order to measure them
properly. I have the area and a centroid with extracted z coordinate
but somehow I cannot make the link between them. The point is that
there are 2 to 4 planes by level that need to be grouped - sorted to
measure them!? Is this clear? Seams simple but somehow not... Please
help thanks.