\(\mathbf{r}(x,y) = x\,\mathbf{i} + y\,\mathbf{j} + z(x,y)\,\mathbf{k}\) \(\mathbf{r}_x = (1, 0, z_x)\) \(\mathbf{r}_y = (0, 1, z_y)\) \(\mathbf{N} = \mathbf{r}_x \times \mathbf{r}_y\)