Gravity disturbances in regions of negative heights : a reference quasi-ellipsoid approachVajda, Peter; Ellmann, Artu; Meurers, B.; Vanicek, P.; Novak, Pavel; Tenzer, RobertStudia geophysica et geodaetica2008 / 1, p. 35-52 : ill https://link.springer.com/article/10.1007/s11200-008-0004-4