Analogous to astrogeodetic methods, the geoid undulation may be determined along profiles by integrating the horizontal components of the gravity vector estimated by airborne GPS inertial navigation system. This has the advantage of obviating large regional surveys of the vertical gravity component that are required in the usual boundary value problem approach. Two data sets were tested, one over the Canadian Rocky Mountains and one over the North Atlantic Ocean. Comparisons with the Canadian geoid model and an Arctic geoid model, respectively, showed better than 10 cm (SD) agreement in the relative geoid, with resolution judged to be between 10 and 20 km. The geoid estimation was found to be sensitive to the curvature of the geopotential field in the mountainous region, and, in the ocean area, the estimates clearly revealed the difference between the geoid and the mean sea surface at this resolution and accuracy.