Geospatial Data Acquisition and Measurement
Geospatial Data Acquisition and Measurement
This domain examines how field observations and sensor data become reliable coordinates: instrument choice, network design, error behaviour, and adjustment.
Core rules and concepts
- Accuracy is closeness to truth; precision is repeatability. Systematic errors bias every observation, random errors scatter, and blunders must be detected and removed.
- Total stations measure horizontal and vertical angles and slope distances. Resection fixes an occupied point from known points; traversing establishes new control; leveling establishes height differences.
- GNSS static baselines are processed relative to control; RTK and network RTK provide real-time positions but depend on correction integrity, satellite geometry, and fixed ambiguity status.
- Redundancy supports least-squares adjustment and blunder detection. Weights should reflect observation uncertainty, commonly proportional to 1/σ².
- Calibration covers prism constants, EDM offsets, level collimation, and GNSS antenna heights.
Worked example
A closed five-sided traverse has observed interior angles summing to 540°00′45″. Theoretical sum = (5−2)×180° = 540°. Misclosure = +45″. With equal weighting, correction per angle = −45″/5 = −9″. After adjustment, bearings and coordinates are computed. If the linear misclosure is 0.12 m over a 600 m perimeter, relative misclosure is 0.12/600 = 1:5000, which would fail a 1:10,000 specification.
Traps
- Calling a biased instrument precise and therefore accurate.
- Omitting antenna height, prism constant, or instrument height from records.
- Distributing angular misclosure by side length when the adjustment model is equal-angle.
- Accepting an RTK fix without checking repeatability, residuals, or a known point.