Editor's note: This article is focused on the scanning device that Master Plan uses. There are other forms of this technology that may vary.
Proper operation of a laser scanner is a very crucial part of the scanning process. While watching a scanning technician in action you may wonder why he is doing certain things. Let us take a deeper dive into the technology that goes into laser scanning and explain how careful procedures will translate into better quality scans.
A laser scanner is equipped with a lidar sensor. The way lidar works is that a sensor emits many laser beams in all directions. The sensor calculates the time it takes for the reflected light to return. All the software has to do is multiply that time with the speed of light and divide it in half. Now it has the distance between one point of the building and the scanner. By scanning millions of points the scanner forms a point cloud of the building. This point cloud is an exact copy of the building made out of millions of little dots.
Some laser scanners rely solely on lidar. These scanners can only scan from one location at a time. Later someone manually aligns dozens of these scans on the computer to create a complete building. This workflow is tedious and time consuming. This is where something called SLAM becomes helpful. SLAM stands for simultaneous localization and mapping. SLAM enables a scanner to be moved around during a scan and know its relative location. Using certain algorithms the scanner compares what it sees now with what it captured previously. These comparisons tell the scanner how much it moved from where it was before. With the advantage of SLAM scanners can now capture an entire house in one scan.
How does this affect the scanning process? Imagine if a scanning technician were to scan one room, cover the sensors and move to another area. Now the scanner has no idea of where it is. The scanner needs a constant feed of information to know its position. For this reason the scanning technician will prop open as many doors as possible. The scanner needs to have constant information for the transition from room to room.
For the best quality scan no people should be present. Moving objects create inconsistent surroundings thereby compromise the automatic stitching of scans. If it is impossible to remove everyone from the areas needing to be scanned the scans will still be able to process. The walls and furniture provide enough information for the software to filter out the moving information. This is another reason why it is best for doors to be propped open. Any movement that can be prevented is beneficial.
One thing that may look unusual is the path the technician will walk while scanning. He will purposely cross over his previous trajectory and walk in small overlapping circles when scanning large open areas. The reason for this is also because of SLAM. After the scan is complete the files are uploaded to a software that processes the data. Errors that happen during scanning are corrected during this process. The software does this by applying the SLAM algorithms to the entire scan. Therefore if areas were walked through twice the two times can be compared with each other. If there was a buildup of small shifts during the scan the software can correct it from the comparison.
Laser scanners are made of a complicated combination of hardware and software. Understanding how these work and relate to each other helps scanning technicians utilize this technology to create perfect point clouds.
