Relative vs absolute accuracy
Two different numbers get called 'accuracy', and confusing them causes most disappointment:
| Type | What it measures | Typical SLAM value |
|---|---|---|
| Relative (local) | Correctness of distances within the cloud | 1–3 cm |
| Absolute | How well the whole cloud sits on your CRS | Depends on control / georeferencing |
For BIM and as-built, relative accuracy usually matters most. For deliverables tied to a coordinate system, absolute accuracy depends on how you georeference — see the SLAM workflow guide.
What degrades SLAM accuracy
- Long, open, featureless runs — nothing for the algorithm to lock onto, so drift grows.
- No loop closure — straight-line paths that never return accumulate error.
- Fast or jerky motion — outpaces the IMU and LiDAR refresh.
- Highly repetitive geometry — identical corridors confuse place recognition.
How to improve SLAM accuracy
Technique often matters more than the hardware:
- Walk closed loops — return to your start and cross earlier paths to trigger loop closure.
- Add control points — surveyed targets let you georeference and check the cloud.
- Move smoothly at a steady pace; slow down at turns.
- Keep overlap between passes so the algorithm always has shared features.
SLAM accuracy vs other methods
| Instrument | Typical accuracy |
|---|---|
| Handheld SLAM | 1–5 cm |
| Terrestrial laser scanner | 1–6 mm |
| Total station | 1–3 mm |
If you need millimetres — control, deformation, forensics — SLAM is the wrong tool; use a terrestrial scanner or total station instead.
Frequently Asked Questions
How accurate are handheld SLAM scanners?
Handheld SLAM scanners typically achieve 1–5 cm accuracy, with relative (local) accuracy often 1–3 cm on a well-executed scan with good loop closure.
Why does my SLAM point cloud drift?
Drift comes from accumulated motion error on long, open or featureless paths without loop closure. Returning to already-scanned areas lets the algorithm realign and distribute the error.
Can I improve SLAM accuracy?
Yes — walk closed, overlapping loops, move smoothly, and add surveyed control points for georeferencing. Technique often improves results more than changing hardware.
Is 1–3 cm accuracy good enough for BIM?
For most BIM and as-built deliverables, yes — 1–3 cm meets typical tolerances. For millimetre control or deformation monitoring, use a terrestrial laser scanner or total station.