A surveying instrument or measurement taken looking backward to a previously established point, used to check accuracy and direction.
From 'back' + 'sight,' originating in surveying terminology around the 17th century. It describes the practice of looking back at a known point while surveying land.
Surveyors use backsights like breadcrumbs—they constantly look back at points they've already measured to make sure they haven't drifted off course, which is why ancient civilizations could build such perfectly straight roads and aqueducts.
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