This question popped into my mind as I was re-reading the “Senjaku hongan nembutsu shu” by Shinran Shonin’s teacher, Honen (1133-1212).
Its title in English is “Passages on the Nembutsu Selected in the Primal Vow,” and it is an explanation of the “Exclusive Nembutsu” that Honen was propounding. Honen also looked up to someone as teacher and master, but that person was from more than 500 years earlier: Shan-tao (613-681, pronounced Shandao).
In addition to declaring that he relies solely on Shan-tao as his master, near the end of the “Senjakushu,” Honen calls Shan-tao’s “Commentary on the Contemplation Sutra” a “Guide to the Western Direction” or, in Japanese, “saiho shinan.”
There is actually an ancient Chinese device called a “shi-nan” (in the Japanese pronunciation) that helped determine direction. It was a kind of compass that would attach to a chariot and point south — the character “shi” means “to point” and “nan” is one of the readings for the character for “south.”
So if you knew which direction you wanted to go, you could orient yourself to the “shinan,” much like we might use a compass now, though in modern times our compasses use north as the main direction to orient against.