The relationship between the wave tilt and the local surface impedance for ground waves propagating over a nonhomogeneous surface has been studied. It is shown that a sufficient condition for their equivalence is that the normal (vertical) electric field and the tangential (horizontal) magnetic field be simply related by the intrinsic impedance of free space. The “phase quadrature” airborne method of measuring the imaginary part of the wave tilt is modified to allow measurement of the imaginary part of the surface impedance. A second method for airborne measurement of the wave tilt is also proposed. It uses two orthogonal electric dipoles lying in the plane of incidence, oriented so that errors resulting from small mechanical misalignment are second order. Both the magnitude and imaginary part of the wave tilt can be measured. The system is best suited for airborne reconnaissance mapping of poorly conducting areas, giving basic information concerning the electrical properties and structure of the uppermost few meters of ground. The basic instrumentation is described but the tasks of development and testing remain.