What is a Backsight (BS) used for in surveying?

A Backsight (BS) in surveying serves a crucial role in establishing a reference point for elevation as well as for the line of sight of the surveying instrument. When setting up a survey, the backsight reading provides a baseline from which the surveyor can measure the relative differences in elevation. By aiming the instrument at a known point and taking a backsight reading, the surveyor can accurately set the elevation of the instrument's line of sight. This is an essential step in ensuring that subsequent measurements are accurate, as it allows the surveyor to establish a level reference for determining the elevations of other points in the area.

The other options do not accurately reflect the purpose of a backsight. While establishing the final elevation, determining distances, and calculating areas are important aspects of surveying, they are not specific functions of a backsight. Instead, a backsight is fundamentally utilized to ensure that the instrument is properly oriented at the correct elevation for accurate leveling and measurement in subsequent surveying tasks.