That you base the y value on the result of a concept called "Barycentric Coordinates". If you're using a grid of quads,

it's easy to compute. (Given that the area of a right-angled triangle is 0.5h*b)

It revolves around weighted coords of the triangle (A, B, C) that currently surround your camera (P).

Basically, the weight of A (wA) is

area(B.xz, P.xz, C.xz) / area(B.xz, A.xz, C.xz)(the surrounding triangle) / (the opposing triangle to the triangle-point in question (here: A))

So you can add together

A.y*wA + B.y * wB + C.y * wCto get the resulting P.y.