Jacob's staff

With the demise of the cross-staff, in the modern era the name "Jacob's staff" is applied primarily to the device used to provide support for surveyor's instruments.

The original Jacob's staff was developed as a single pole device, in the 14th century, that was used in making astronomical measurements.

It was first described by the French-Jewish mathematician Levi ben Gerson[4][5] of Provence, in his "Book of the Wars of the Lord" (translated in Latin as well as Hebrew).

[7] Its invention was likely due to fellow French-Jewish astronomer Jacob ben Makir, who also lived in Provence in the same period.

[11] Shen was an antiquarian interested in ancient objects; after he unearthed an ancient crossbow-like device from a home's garden in Jiangsu, he realized it had a sight with a graduated scale that could be used to measure the heights of distant mountains, likening it to how mathematicians measure heights by using right-angle triangles.

In the 15th century, the German mathematician Johannes Müller (called Regiomontanus) made the instrument popular in geodesic and astronomical measurements.

[10][15] As a navigational tool, this instrument was eventually replaced, first by the backstaff or quadrant, neither of which required the user to stare directly into the sun, and later by the octant and the sextant.

[16] This actually increased the accuracy of the instrument, as the navigator no longer had to position the end of the staff precisely on his cheek.

[18] Backstaves were no longer allowed on board Dutch East India Company vessels as per 1731, with octants not permitted until 1748.

[19] It also has a screw base and occasionally a ball joint on the mount, and is used for supporting a compass, transit, or other instrument.

These include: Some devices, such as the modern optical targets for laser-based surveying, are still in common use on a Jacob's staff.

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.