NICET - Highway Construction Inspection Practice Exam

Which formula is used to calculate the height of instrument?

1. TP = HI - FS

2. HI = BM + BS

3. ΣBS - ΣFS = ΣLast RL - ΣFirst RL

4. HI = FS - BM

The correct answer is: HI = BM + BS

To calculate the height of the instrument (HI) in surveying, the formula HI = BM + BS is used, where BM represents the benchmark elevation and BS represents the backsight reading taken from the instrument to a known point. This formula is critical in surveying because it establishes the instrument's height relative to a known reference point (the benchmark) and incorporates the measured distance from the instrument to the benchmark. When you have a known elevation for a benchmark and have taken a measurement (backsight) towards it, adding the backsight value to the benchmark elevation allows surveyors to find the height of the instrument accurately. This height of the instrument is essential for determining new elevations and conducting further measurements in the surveying process. Understanding this formula and its components is key for surveying tasks, as it enables reliable calculations for creating accurate topographic maps and performing construction layout work.