Interpolate / Smooth Surface Point cloud

I have a matrix of points (x,y,z) which describe a flowpath, however these points are scattered and not uniformly spaced. It's a "bendy-tube" oriented in the z direction, so the points don't describe a "function" of x,y. How can one interpolate the points at a specific z value so I can calculate the area? I can calculate the area at z=0 using convhull because there are many data points at z=0, but how could I do it for a specific z value?

I've tried triscatteredInterp, griddata, griddedinterpolant, and interp3. My end goal is to get area as a function of the z direction. Griddedinterpolant(x,y,z) makes an interpolant, but I don't know how to get the data from it.

[Matt J]

I can calculate the area at z=0 using convhull because there are many data points at z=0, but how could I do it for a specific z value?

So the cross-sections of the tube at every z are convex?