backsight

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backsight

[′bak‚sīt]
(engineering)
A sight on a previously established survey point or line.
Reading a leveling rod in its unchanged position after moving the leveling instrument to a different location.
(navigation)
A marine sextant observation of a celestial body made by facing 180° from the azimuth of the body and using the visible horizon in the direction in which the observer is facing.

backsight

In surveying, a sight on a previously established survey point or line.