foresight

In levelling work, a foresight is a rod reading taken on a point ahead of the surveyor's current instrument position. The surveyor sets up a level (or transit) at a station, reads the backsight on a known point behind, then turns and reads the foresight on a new point forward. This forward reading, combined with the backsight and the height of instrument, allows the surveyor to determine the elevation of the foresight point and move the level progressively across a site.

The foresight point itself becomes the backsight point for the next setup. This leapfrog method allows surveyors to build chains of elevation measurements across terrain, connecting known benchmarks to new points. The accuracy of the final elevation depends on minimizing errors in each rod reading, keeping backsight and foresight distances roughly balanced, and checking for instrumental errors like collimation drift.

Practical distinctions

Foresights are taken on a levelling rod held vertically on the ground. The surveyor reads the height of the horizontal cross hair against the rod's graduations, typically to the nearest 0.01 foot or 1 millimetre depending on the instrument and rod type. A turning point (TP) is any point where both a backsight and foresight are taken; a bench mark (BM) is a fixed reference point with a known elevation. Intermediate sights, taken between backsights and foresights, establish heights of features but do not anchor the next setup.

The term 'foresight' reflects its directional sense: the sight looking forward into unmeasured terrain. In older literature and some regions, it is called a 'foreshot' or 'forward sight'. The backsight-foresight pair forms the backbone of differential levelling, the standard method for establishing accurate vertical control on engineering and construction projects before setting out or checking finished grades.