Measurement and Data Acquisition
Precision is not accuracy
Precision is repeatability - how tightly repeated observations cluster. Accuracy is closeness to truth. An instrument reading consistently in the same wrong direction is precise and inaccurate; that pattern signals a systematic error such as an uncorrected prism constant, a wrong atmospheric setting, or a bad instrument height. Random errors are small and unavoidable; blunders are mistakes and must be detected and rejected.
Know the standard corrections: prism constant, atmospheric correction from temperature and pressure, curvature and refraction (combined 0.0206·M² ft, with M in thousands of feet), grid-to-ground scale factor, and instrument and level calibration. For levels, HI plus or minus rod readings, loop closure checked against a class standard.
For GNSS, distinguish autonomous, DGPS, RTK, and network RTK (VRS). Horizontal accuracy generally exceeds vertical; satellite geometry (PDOP), multipath, canopy, and initialization time all matter.
Traps
- Mixing grid and ground distances without applying a combined scale factor.
- Forgetting to apply HI, or adding it when the rod reading requires subtraction.
- Reporting more significant figures than the instrument supports.
- Confusing traverse misclosure with the error in a single observation.
- Assuming RTK elevations meet differential-leveling standards.
Rule of thumb
Judge level-loop misclosure against the project class standard, for example 0.05·√M ft where M is the loop length in miles; a loop that closes to 0.03 ft over 4 miles passes that test easily, a 0.4 ft closure over 1 mile does not.
Worked example
A 1,200 ft sight is taken with the rod 1,200 ft away. Curvature and refraction correction = 0.0206 × (1.2)² = 0.030 ft, subtracted from the rod reading because the line of sight curves upward relative to level.