plumb line (Geography Dictionary)

plumb line : A vertical reference line created by suspending a weight, known as a plumb bob or plummet, from a string above the Earth’s surface and allowing it to hang freely in the direction of the pull of gravity. A precursor to the spirit level, plumb lines are used to establish a vertical datum in a wide range of applications, particularly in surveying to determine the nadir of a point in space, and often in combination with an instrument in order to set the instrument precisely over a fixed survey marker.