Surveying

Backsight and Foresight

Level rod readings taken on a point of known elevation (backsight) and on a point to be found (foresight).

In differential leveling, a backsight is a rod reading on a point whose elevation is known; adding it to that elevation gives the height of instrument. A foresight is a reading on the next point; subtracting it from the height of instrument gives that point’s elevation.

Over a whole run, the final elevation equals the starting elevation plus the sum of backsights minus the sum of foresights, which is a quick arithmetic check. Keeping backsight and foresight distances about equal cancels curvature, refraction and any tilt of the line of sight.

How much of the FE Civil exam is Surveying?