An attempt for an Amazon geoid model using Helmert gravity anomalyBlitzkow, D.; Matos, A.C.O.C.de; Campos, I.O.; Ellmann, Artu; Vanicek, P.; Santos, M.C.Observing our changing Earth : proceedings of the 2007 IAG General Assembly : Perugia, Italy, July 2-13, 20072009 / p. 187-194 : ill