Given two arbitrary views of a scene under central projection, if the motion of points on a parametric surface is compensated, the residual parallax displacement field on the reference image is an epipolar field. If the surface aligned is a plane, the parallax magnitude at an image point is directly proportional to the height of the point from the plane and inversely proportional to its depth from the camera. The authors exploit the above theorem to infer 3D height information from oblique aerial 2D images. The authors use direct methods to register the aerial images and develop methods to infer height information under the following three conditions: (i) focal length and image center are both known, (ii) only the focal length is known, and (iii) both are unknown.