Digital Geometry Processing with Discrete Exterior Calculus
cs.columbia.edu
cs.columbia.edu
But you can download his paper on them at http://www.cs.columbia.edu/~keenan/Projects/GeodesicsInHeat/...
(and somewhat more high-brow, but still with a discrete flavour.)
Can't use the (x^n)' = n x^(n-1) on real data! I need to take a derivative of some GIS numbers have to compute a discrete gradient. In fact that's what I did to draw this figure of Puerto Rico
http://www.cs.columbia.edu/~keenan/faq.html
modo and tracing (perhaps by hand)