How Mobile Laser Scanning Works
Author: Dr. Robert Radovanovic, P. Eng., A.L.S., C.L.S.
3D Laser Scanning generally means using a piece of equipment called a scanner to collect a massive, 3D point cloud of a scene. These can include roadways, an industrial area, or the inside of a building. The scanner shoots out laser pulses millions of times per second as it rotates vertically and horizontally. Laser pulses reflect off surrounding surfaces and objects and the scanner measures the time it takes the pulses to come back to it. The distance travelled by an individual pulse is then calculated by multiplying the speed of light by the travel time.
Using all of the calculated distances and knowing the directions the pulses were fired at, the scanner builds up a point cloud of reflection locations. The result is an accurate, high-definition model of the respective scene. Real-world coordinates can also be calculated if the position of the scanner and its orientation is known. This allows for the creation of a 3D model where every point has a position — geo-referencing.
Collection speed is one of the prime advantages of scanning. Scanners capture massive amounts of 3D data quickly and accurately, while traditional surveying efficiency is limited by how fast a surveyor can target and measure points. Laser scan surveys translate to less time spent in the field. This means less disruption to site operations (such as traffic control or shutting down operating areas) and greater safety for everyone involved by keeping people out of harms way. Scanning also enablesdata-mining. This is where features of interest can be pulled out of the 3D point cloud at a later date. This means a scan can be done once and project teams then benefit from the use of data multiple times as a project scope evolves.
Although faster than traditional surveying, static scanners still need to be moved through a scene in order to create a complete point cloud model. Scans must be taken from different positions to avoid shadow-zones. Scanners also have a limited line-of-site range. The accuracy of the data begins to decrease the farther the laser extends. While scanning (typically a 5-10-minute process), the scanner needs to stay absolutely still, so no positional errors occur. Additionally, the scanner positions typically need to be surveyed so that individual scans can be tied together into one cohesive model.
In the case of mobile scanning, the scanner collects the scan cloud while it’s moving. In a typical setup, an accurate positioning system logs the position of the scanner at a rate of several hundred times per second. The positioning system combines GPS with Inertial Measurement instruments. Both the location of the scanner and direction of the lasers while it’s moving are known. The scanner continues to rotate and shoot laser pulses as normal, but now the system can calculate the location and orientation of the scanner at every pulse.
Mobile scanning systems build up 3D point clouds while in motion, which dramatically increases the speed of data capture. In fact, data can be collected as quickly as the vehicle can safely travel.
Additionally, since the positioning system figures out where the scanner is at all times, the scanner itself doesn’t need to travel smoothly, or in a straight line, or any particular way at all. This means mobile laser scanners can be mounted to a range of platforms (such as cars, boats, drones or even utility vehicles) to suit any project scope. Mobile laser scanning applications vary widely, since data can be collected quickly and without getting in the way of operations.
The majority of CorridorMapsTMprojects are collected using mobile laser scanning systems, capturing dense 3D point clouds (usually 1 point per 3 cm) at survey-grade accuracies (typically ±25 mm). High-resolution 360-degree photography is collected at the same time, which provides great situational awareness through streetview-style overviews. Combining photos with laser data allows for dense, accurate point clouds of transportation corridors, which can replace traditional surveying in many cases. Additionally, collecting this data early in the project enables verification of current conditions, which helps improve cost estimation, planning and design.