Earthwork Calculation

I am trying to use cspline to perform an earthwork calculation on a 115 acre site from which I have taken 100 spot elevations, a very rough data set admittedly. The attached follows a previous post by Giraud. I stopped short of performiing an integration of the surface generated by the interp because I was not sure how to interpret the results; how to relate the scale to the actual data, i.e., the xy grid. Further, when I generated a mesh using CreateMesh, I get a gruesome figure.