SLAM Acquisition
Last updated
Last updated
While most SLAM data is captured indoors, without GNSS reception, it is important to begin your data acquisition outside to allow the system to receive GPS time and record an initial position to globally reference your indoor scan. Spend at least 2 minutes outdoors in an open environment at the beginning of the scan to obtain quality GNSS data.
Georeferencing will be more robust if your scan has several periods of GNSS reception at different places intermixed with periods of indoor scanning.
Power on the rover outside.
Connect to the system via the web interface, using a mobile phone or tablet.
Press Start Rover, and wait for Time status to become FineSteering.
Capture will be enabled after the Time status FineSteering is achieved. This can be confirmed in Messages, where the message "GNSS determined accurate time, allowing capture to be enabled." will appear.
Proceed indoors and scan.
The Time status can degrade afterwards (e.g. after entering a building). This will not impact the ability to start capturing data when FineSteering has been achieved at least once after Rover has started.
Power on the system outside and connect to the web interface.
Set the camera trigger interval to 120 seconds if you are not interested in recording camera data.
Ensure the antenna offsets and IMU orientation are correct for a SLAM setup.
Allow the system to track satellites for approximately 2 minutes.
Click "Start recording" to allow the system to begin recording lidar and navigation data.
Proceed indoors to scan.
Scanning Motion
Keep the unit steady and with a good view of it's surroundings for the first 10 seconds once you see LIDAR data come in. This is to build a good initial map for SLAM to orient itself in. As you move indoors, move the unit steady and try to cover all surfaces.
Continue performing a smooth motion while you collect LiDAR data. Be aware that LiDAR pulses may reflect off of glass windows and doors, resulting in an undesirable georeferencing of the lidar pulse.
Small rooms may require extra scanning, as many of the LiDAR pulses will not register as a return due to the minimum range of the scanner; with the Velodyne VLP-16 and Hesai XT32, this minimum range is about 1 meter. As well, ceilings and floors will require more deliberate scanning than features at eye-level, as the field of view on the LiDAR scanner is narrow and will not typically receive many of these points.
Loop Closure
Loop closure refers to the SLAM algorithm adjusting it's current map and trajectory based on the appearance of a feature that was previously scanned. As you scan in a GNSS denied environment, the position as determined by SLAM drifts from the true position. Loop closure can effectively reduce this drift by adjusting the current map to a previous map which was created with less drift.
Consider loop closure when scanning. When scanning large, open, indoor spaces, try to walk in a circle, so that the end of the scan corresponds to the beginning of the scan. If you scan a hallway or mostly linear indoor space, walk down and back through the hallway.
Other best practices:
Scan when the building is mostly unoccupied, so that people do not appear in the scan.
Exit the building when feasible to obtain additional GNSS fixes and limit trajectory drift.
Open doors prior to the scan. This way the "state" of the door (open or closed) is fixed during the scan and the door does not appear as both opened and closed in the point cloud.
Scan with a friend! It's helpful to have someone who can open doors prior to you entering the space, as well as close doors when you exit the space.
If you plan on matching your SLAM scan to an aerial scan, try to get good lidar coverage on features that are visible in the aerial scan. For instance, if you exit the building to get a GPS fix, try to get some lidar coverage on the building walls and roof, if possible.
At all times, and especially at the starting location, the LiDAR unit should have good visibility of the world around it. Avoid confined spaces. Do not occlude the LiDAR unit's field of vision.
The recommended position for mounting the LiDAR is straight, i.e. such the unit has a horizontal 360° view of the surroundings. If acquisition is performed by carrying, the LiDAR may be gently swung in order to increase coverage.
Either completely avoid revisiting the same areas, or revisit them thoroughly, making sure there is decent overlap of the same area between the old and the new parts of the trajectory.
