a Jacob's staff. These include: Cross-head, cross-sight, surveyor's cross or cross Graphometer Circumferentor Holland circle Miner's dial Optical square Surveyor's sextant Surveyor's target Abney level Some devices, such as the modern optical targets for laser-based surveying, are still in common use on a Jacob's staff. In geology In geology, the Jacob's staff is mainly used to measure stratigraphic thicknesses in the field, especially when bedding is not visible or unclear (e.g., covered outcrop) and when due to the configuration of an outcrop, the apparent and real thicknesses of beds diverge therefore making the use of a tape measure difficult. There is a certain level of error to be expected when using this tool, due to the lack of an exact reference mean for measuring stratigraphic thickness. High-precision designs include a laser able to slide vertically along the staff and to rotate on a plane parallel to bedding. See also Backstaff Cross of St James Pilgrim's staff Tacheometry As a symbol in Scouting: 5th World Scout Jamboree References Further reading Levi ben Gerson and the Cross Staff Revisited, Bernard R Goldstein 14th-century introductions Angle measuring instruments History of astronomy Historical scientific instruments Navigational equipment Song dynasty Surveying instruments Celestial navigation