A plumb line is a reference line guided by a string or cord weighted at the end with a large weight known as a plumb bob . It is used to create a reference line to create vertical lines, where a spirit level may be impractical or difficult to use.
The line has in every point the same direction as that of the force of gravity of the Earth; thus, an object dropped on the surface of the Earth tends to follow this line.
Strictly speaking the plumb line is not straight: it is curved mainly in the North-South direction (approximately 0.2" per km of elevation). This is mathematically connected with the fact that gravity diminishes when going from pole to equator. The curvature is additionally affected by rough topography.
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