Volume between 2 sets of nonuniform 3-d data

Hi,

For a project I'm working on, I have 2 sets of 3-d data that I used to generate 3-d surface plots. The first data set is the surface of a polyethylene piece before wear and the second data set is the surface of the same piece after wear. Since these polyethylene pieces were machined, the surface is not completely smooth and have ridges as part of the machining. I've overlayed the two graphs so that the edges are about matched up and so I can see the before/after change (the before has a concavity in the center and the after has a convexed center).

Does anyone have any suggestions as to how I can find the volume between the concaved and convexed regions? I need it within a specific range (x,y,z) because there are other regions that are irregular. But I would just like to focus on the specific set of data points that form the concavity/convexity.

[Walter Roberson]

Joan, it would help if you were to give feedback as to why the previously suggested methods were not suitable.

http://www.mathworks.com/matlabcentral/answers/8286-calculating-volume-under-a-3-d-set-of-non-uniform-data-points