 His maps also included schematic information on geography, converting areas of administrative districts (intendancies) using proportional squares.The U.S. was keen to see his maps and statistics on New Spain, since they had implication for territorial claims following the Louisiana Purchase.Later in life, Humboldt published three volumes (1836–39) examining sources that dealt with the early voyages to the Americas, pursuing his interest in nautical astronomy in the fifteenth and sixteenth centuries.