vertical plane through P0 in the direction u
the curve where that plane meets the surface
tangent line to that curve at P0
level curve of h through P0, where h = 2974 ft
horizontal ray, against which α is measured
P0, the point of tangency
surface z = h(x,y) = 3000 − 0.0004x² − 0.001y² · drag the picture to rotate
α is the angle the tangent line makes with the horizontal, and tanα = Duh(P0): the rate of change of h along u is the slope of that line.