References
----------
 Hinze, W.J., Aiken, C., Brozena, J., Coakley, B., Dater, D., Flanagan, G., Forsberg, R., Hildenbrand, T., Kaller, G.R., Kellogg, J., Kucks, R., Li, X., Mainville, A., Moring, R., Pilkington, M., Plouff, D., Ravat, D., Roman, D., Urrutia-Fucugauchi, J., Veronneau, M., Webring, M., Winester, D. 2005. New standards for reducing gravity data: the North American gravity database. Geophysics, 70, J25-J32.