Precision and accuracy are two different measurement quality characteristics that are often confused. In surveying, understanding both parameters is essential for ensuring quality results.
Accuracy vs precision:
- Accuracy - how close a measurement is to the true value
- Precision - how repeatable measurements are (dispersion)
- Measurements can be precise but inaccurate (systematic error)
- Ideal measurements are both accurate and precise
Expressing precision:
- Standard deviation (σ) - statistical dispersion measure
- Root mean square error (RMS)
- Confidence interval - 68%, 95%, 99%
- Absolute error - mm, cm, m
- Relative error - ppm (parts per million)
Precision requirements in surveying:
- Cadastral surveying - ±5 cm for boundary points
- Topographic surveying - depends on scale
- Engineering geodesy - up to ±1 mm in specific works
- Deformation measurements - sub-millimeter precision
Factors affecting precision:
- Instrument quality - class and calibration
- Measurement method - method selection
- Environmental conditions - temperature, wind, visibility
- Operator skills - experience and care
- Geometry - satellite configuration, angles
Precision control:
- Repeated measurements on control points
- Traverse closure error analysis
- Comparison with higher precision data
- Statistical data analysis
Instrument precision:
- GNSS RTK - 1-3 cm horizontally, 2-5 cm vertically
- Total station - 2-5 mm + 2ppm for distances, 1-5" for angles
- Level - 0.3-3 mm/km
- LiDAR - 1-3 cm for scan points
Ensuring measurement precision is the surveyor's main task, directly affecting work quality and legal validity.