There is a note on cross-staffs and their use in The Art of Navigation in England in Elizabethan and Early Stuart Times by Prof. David Waters (Hollis and Carter, 1958).
By the way, David Waters gave the Andrew Somerville Memorial Lecture at the last British Sundial Society annual meeting. Summarising in a sentence, the Crusades made the Italian city states rich through sea transport to the Holy Land, resulting in the Renaissance, and all was made possible by the invention of the mariner's compass facilitating all-season navigation of the Mediterranean. -- Frank Evans
