Plumb line, also called plummet, is a string or line with a weight attached to one end. The weight, called a plumb bob, keeps the line straight up and down. Bricklayers and stonemasons use plumb lines as vertical guides in building walls. Surveyors and engineers use plumb lines to set sighting instruments called transits and theodolites over specific points. Some theodolites have an optical plummet, a special telescope device that provides more precise placement over a point.
Plumb lines are also used to determine the depth of water or of excavations. But today, crews on most ships use instruments called fathometers and sonars to measure the depth of the water.
See also Surveying (Surveying tools) .
