Differential leveling transfers a known elevation from a benchmark to one or more unknown points, and the whole method rests on a single bookkeeping rule. A backsight is a reading taken on a point whose elevation is known, and adding it to that elevation gives the height of instrument, the elevation of the line of sight itself. A foresight is a reading taken on a point whose elevation is wanted, and subtracting it from the height of instrument gives that elevation. Backsights are added, foresights are subtracted, and every level circuit is that pair repeated. The name height of instrument causes more trouble on the FE Civil exam than the arithmetic does. It is the elevation of the horizontal line of sight above the datum, not the height of the tripod above the ground, and a question that supplies an instrument height in metres above the point is usually testing exactly that confusion. Turning points are the other place to slow down: a turning point is read as a foresight from one setup and then as a backsight from the next, so its elevation must be established before the instrument moves. When a circuit closes back on its starting benchmark, the difference between the computed and known elevation is the misclosure, and it is distributed across the setups in proportion to distance.
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Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.
FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES. All practice questions are original and generated for study purposes based on publicly available exam specifications.