Partial derivatives for surfaces

[noamross]

Curious if we could have (partial) derivatives for s(x,y) or similar interaction terms.

[gavinsimpson]

By partial derivative do you mean the derivative in x holding y constant etc?

I have notes on how this is done with finite differences and the more general derivative of x,y, I just need some time to code it up. Might be something I get to between jobs while in quarantine in Denmark later this month.