The Collaborative International Dictionary
Forestaff \Fore"staff`\, n. (Naut.)
An instrument formerly used at sea for taking the altitudes
of heavenly bodies, now superseded by the sextant; -- called
also cross-staff.
--Brande & C.
Wiktionary
n. 1 (context nautical English) An early navigational instrument, consisting of a wooden rod with a sliding crosspiece, used for measuring the altitude of a star; it developed into the sextant 2 A surveyor's instrument for measuring offsets.
Usage examples of "cross-staff".
They wore rich accoutrements, and the hands of one were frozen to a cross-staff holding a silver boar's head on a red field.
For determining his approximate position, a rough estimate at best, he relied on the sun and stars while using a cross-staff he'd fashioned of two slivers of wood cut from the paddle.
He took a sighting of the sun with his cross-staff and set a course with the compass.