How do we know if SLAM is drifting or broken?

Using control points during a scan helps improve Point cloud accuracy. This is fully documented here. However, the use of control points cannot always save twisted or tilted scans. This article describes when control points can improve your scanned data and in which scenarios they cannot.

The NavVis VLX uses an algorithm called Simultaneous Localization and Mapping (SLAM). You can find more information on SLAM here:

• https://www.navvis.com/technology/slam

• https://www.navvis.com/blog/how-slam-affects-the-accuracy-of-your-scan-and-how-to-improve-it

SLAM Drift

During NavVis Post-Processing, control points are used to eliminate drift within the walked trajectory during the scan.

A drifting SLAM is more common in outdoor environments due to the size and because the scanned loops are often made bigger than when indoors. Drift means that the scans were correctly matched to the map, but each scan has some distortion due to imperfect sensors, which leads to accumulating errors over time. Using surveyed control points during post-processing will generally be able fix a drifting Dataset.

Drifts can be recognized by:

• A consistent and continuous trajectory.

• Long straight walkways or corridors being slightly tilted along the whole distance (not in one exact spot).

• Larger loops not closing in the same location.

SLAM Break

In more challenging environments the SLAM can break. This means that during scanning the maps are matched incorrectly, e.g. because of insufficient distinctive features in the environment or in very noisy environments. Using control points with this type of issue will not help.

In most cases a break can be recognized by:

• A jumping trajectory.

• Duplicated structures like buildings.

If the SLAM breaks, there is a possibility it can be fixed during post-processing, but in extreme cases it cannot. In such situations, control points will be not able to correct the breaking trajectory since there is no data on how to distribute the difference in distances between the control point coordinates and the scanned control point coordinates.