1. Fiala, F. 1957.Mathematische Kartografie, Edited by: Zappe, A. 54–78. Berlin: Verlag Technik. One author gives twenty-five possible solutions
2. Robinson, A. H. 1960.Elements of Cartography, 2nd ed., 328New York: John Wiley & Sons. Compare the use of Zm rather than m in the measurement of angular distortion on a map projection as defined by Tissot's indicatrix; see