What would you do if you only had 1 Dimension?
You would do (y2-y1)/(x2-x1) with x2 being the nearest x to x1. Do exactly the same with multiple Dimensions.
- Any given Point (x,y) will be on/within a triangle of neighboring points A, B, C.
- Find the Points A B C of that triangle and then identify the gradient (hint: normal vector gets you there) in 3D of the area ABC.