CST Level 1 Land Surveyor Certification Practice

What does backsight measure during differential leveling?

1. The vertical distance to an unknown elevation

2. The vertical distance to a known elevation

3. The horizontal distance to the benchmark

4. The lateral distance between two benchmarks

The correct answer is: The vertical distance to a known elevation

The correct understanding of backsight in the context of differential leveling is that it measures the vertical distance to a known elevation. During the leveling process, backsight reading is taken through the leveling instrument (such as a level or transit) to a point with a known height, often referred to as a benchmark. This reading establishes a reference point for determining the elevation of other points that may be unknown. When conducting differential leveling, the process involves taking a backsight reading first to establish the height of the instrument above the known elevation. This is essential because it allows the surveyor to calculate the differences in elevation between the benchmark and any other point being surveyed. Hence, the role of the backsight is pivotal in accurately determining subsequent elevations relative to that known point. In contrast, the other options misrepresent the function of the backsight; for example, measuring vertical distance to an unknown elevation does not accurately describe backsight, as backsight is specifically linked to known points. Similarly, the options relating to horizontal and lateral distances do not pertain to the backsight function in differential leveling.