Birdi's geotechnical monitoring reports use three layers of accuracy. Together they tell you how reliable the reported movement is.
1. Ground control accuracy
Ground control points (GCPs) tie your map to real-world coordinates during processing. The accuracy of this fit is recorded in the processing report that comes with each dataset. Check it first: a large residual on a single GCP may mean the point is damaged, has been disturbed, or was surveyed wrong.
2. Tagging accuracy
GCPs and prisms are tagged by hand in the captured imagery, which adds a small amount of placement bias. As a guide:
GCPs: about 20–30 mm
Prisms (low-altitude monitoring flights): about 3–4 mm
3. Movement accuracy
Prisms are used as checkpoints, not as control. This means they measure movement independently of the data used to position the map. The horizontal and vertical movement accuracy for each capture is shown at the top of your monitoring report.
Tolerance levels
Tolerances set how much movement causes a prism to be flagged, and you configure them for each site. Every change is recorded with the user and time of the change. The values in use are saved on each report, so editing a tolerance doesn't change reports you've already generated. Check your units carefully when you enter tolerances (monitoring tolerances are usually in millimetres).
Factors that affect accuracy
Image overlap and coverage of the capture
Condition and survey quality of your GCPs
Flight height (lower flights give finer tagging accuracy)