What is a geomantic compass?

It is equivalent to "undergraduate, plus graduate students, plus doctoral credits". A good compass has more than 60 floors, and each floor has a book volume equivalent to 6 to 8 credits, that is, there are more than 60 such books, and more than 20 books are not reflected in the compass, but they need a compass to operate. Zhong Yiming's Kong Xuan, for example, is one of the six books, weighing about two kilograms.