Measurement Technology and Data Collection
This domain covers instruments, corrections, and field data quality. Questions often hinge on a small correction: prism constant, atmospheric scale error, curvature and refraction, or a GNSS fix that was never truly fixed.
Total station and EDM
- Correct for prism constant, atmospheric conditions, and curvature and refraction. Combined curvature and refraction is approximately 0.0206 D² ft when D is in thousands of feet.
- Instrument checks include collimation error, trunnion-axis error, vertical-circle index error, and the optical plummet or laser plummet.
- Reflectorless measurement depends on surface color and angle; it is unsuitable for precise deformation or long distances.
- Reciprocal vertical angles cancel curvature and refraction and are preferred for long or steep shots.
GNSS and data collection
- RTK and network RTK require a fixed solution, adequate satellite geometry, and a clear sky view. PDOP, multipath, canopy, and nearby reflective surfaces degrade results.
- GNSS heights are ellipsoid heights; convert with a geoid model to NAVD88.
- Field codes and feature codes should be unambiguous, with redundant observations, check shots to known control, and metadata recording instrument, date, datum, and observer.
Worked example
A total station measures a 1,500-ft shot. Curvature and refraction = 0.0206 × (1.5)² = 0.046 ft, which must be applied to a single observation; reciprocal observations make it cancel. Separately, the EDM has a +3 ppm scale error: over 1,500 ft the error is 3 × 10⁻⁶ × 1,500 = 0.0045 ft. The curvature term dominates, but both belong in a defensible reduction.
Traps that cost marks
- Entering the prism constant with the wrong sign or using the wrong prism type.
- Applying curvature and refraction twice, or applying it to reciprocal observations.
- Accepting a float solution as if it were fixed.
- Ignoring antenna height or using the slant height as vertical.
- Recording coordinates without the datum, epoch, or geoid model.