A seamed rectangular element is introduced, on each of whose sixteen triangular panels the function is quadratic; the element is internally continuously differentiable, is specified by value and gradient data at its vertices, and by the requirement of linearity of normal component of the derivative along its edges, and has no spare degrees of freedom under these requirements. Its use in conjunction with the Marlow–Powell quadratic contouring algorithm facilitates the economical production of high-quality contour maps.