Measurement and Data Collection
Measurement and Data Collection
The PS exam tests field measurement quality: instrument use, error propagation, correction application, and GNSS. Expect tape corrections, leveling, total-station checks, and questions on when GNSS is appropriate.
Error types
Systematic errors are consistent and correctable (temperature, tension, sag, collimation). Random errors scatter and are treated statistically. Blunders are mistakes and must be rejected. Precision describes repeatability; accuracy describes closeness to truth.
Tape and distance corrections
For a steel tape, temperature correction: Ct = L α (T − T0), with α ≈ 6.5×10⁻⁶/°F. Tension correction: Cp = (P − P0)L/(AE). Sag correction is negative: Cs = −w²L³/(24P²). Slope correction reduces horizontal distance: H = S cos θ.
GNSS and leveling
RTK GNSS must show a fixed solution; float or autonomous solutions are not survey-grade. PDOP, multipath, and canopy degrade accuracy. GNSS heights are ellipsoidal; orthometric heights require a geoid model. Differential leveling controls vertical work; keep sight distances balanced to cancel refraction and curvature.
Worked example
A 100.00-ft steel tape standardized at 68°F is used at 92°F. α = 6.5×10⁻⁶/°F. Ct = 100.00 × 6.5×10⁻⁶ × (92 − 68) = 0.0156 ft. The tape is longer than standard, so the measured distance is too short by 0.0156 ft. Corrected distance = 100.00 + 0.0156 = 100.0156 ft. Reason: expansion makes each tape unit longer, so fewer units fit the true distance.
Traps
- Applying the temperature correction with the wrong sign.
- Forgetting that sag correction is always negative.
- Using a float GNSS solution for a boundary corner.
- Treating ellipsoidal height as orthometric.
- Ignoring prism constant or instrument height.