But that area (refer to by some as the surface area as opposed to the planimetric , or projected, planar area) will differ depending on how the 3dfaces area created.
Creating/connecting the 3dfaces gets back to some of the previous discussions on triangulation and TIN creation.
I'm thinking that 10 people asked to calc the 3D area would come up the 10 different answers. It isn't too difficult to calculate A 3d area. Calculating THE 3d area, IMO, a different task all together