Field Data Acquisition and Measurement
Core concepts
Field work requires understanding total stations, EDM, GNSS, differential leveling, traverse design, and error theory. Distinguish accuracy from precision, random from systematic error, and mistakes from errors. Know instrument height, target height, prism constants, atmospheric corrections, calibration, and the difference between horizontal and slope distances. For GNSS, know RTK limitations, multipath, satellite geometry, and the need for a verified control check. For leveling, know benchmark datum, closure, and the effect of refraction and curvature on long shots.
Traps
Questions often test whether a precise but biased result is accurate. Another trap is applying a scale factor when converting grid to ground, or forgetting to correct for temperature and pressure on long EDM shots. Do not confuse a closed traverse with a blunder-free traverse. GNSS elevations are generally less reliable than horizontal positions; do not assume RTK vertical accuracy matches horizontal accuracy.
Worked example
A 1-second angular error displaces a point by about 0.005 ft at 1,000 ft (1 second is about 4.85 millionths of a radian). At 500 ft the displacement is about 0.0024 ft. For a six-sided traverse, interior angles should sum to (6-2) × 180 = 720 degrees; an observed sum of 720°01'12" has 72 seconds of angular misclosure. Rule of thumb: distribute angular misclosure equally among the angles before computing coordinates.