Surveying
In surveying the Jacob's staff, contemporaneously referred to as a jacob staff, is a single straight rod or staff made of nonferrous material, pointed and metal-clad at the bottom for penetrating the ground. 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.
The term cross-staff may also have a different meaning in the history of surveying. While the astronomical cross-staff was used in surveying for measuring angles, two other devices referred to as a cross-staff were also employed.
- Cross-head, cross-sight, surveyor's cross or cross - a drum or box shaped device mounted on a pole. It had two sets of mutually perpendicular sights. This device was used by surveyors to measure offsets. Sophisticated versions had a compass and spirit levels on the top. The French versions were frequently eight-sided rather than round.
- Optical square - an improved version of the cross-head, the optical square used two mirrors at 45° to each other. This permitted the surveyor to see along both axes of the instrument at once.
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Famous quotes containing the word surveying:
“Upon looking back from the end of the last chapter and surveying the texture of what has been wrote, it is necessary, that upon this page and the five following, a good quantity of heterogeneous matter be inserted, to keep up that just balance betwixt wisdom and folly, without which a book would not hold together a single year.”
—Laurence Sterne (1713–1768)
“As for my own business, even that kind of surveying which I could do with most satisfaction my employers do not want. They would prefer that I should do my work coarsely and not too well, ay, not well enough. When I observe that there are different ways of surveying, my employer commonly asks which will give him the most land, not which is most correct.”
—Henry David Thoreau (1817–1862)