Explanation

In surveying, a backsight and an intersight are both readings taken with an instrument such as a level to determine the difference in elevation between two points.

A backsight reading is typically taken at the starting point of a survey, and serves as a reference elevation for all subsequent measurements.

An intersight reading is taken after the starting point, on another point and it measures the difference in elevation between the current point and the starting point.

If the back sight reading is 3.370m and the intersight reading is 2.975m, the difference in level is the difference between the two readings, which is:

3.370m - 2.975m = 0.395m ( or 39.5 cm)