Frequently Asked Questions

Many people use these terms to talk about the same thing. However, as defined above, LIDAR refers to the underlying technology – the measurement physics, while 3D laser scanning is the activity that makes use of LIDAR. A 3D laser scanner employs LIDAR technology to determine the range, or distance to a point. The scanner can also rotate in one or two planes while sensing the angles to the points. This information can then be used to determine the three dimensional coordinates of each captured point.

A mobile LIDAR scanner refers to a laser scanner that is attached to a mobile platform, such as an SUV or a boat. Generally the term MLS refers to terrestrial applications where airborne LIDAR is the term used for data collection that occurs above the earth. Technically both are mobile or kinematic, but the terminology has evolved over time to have these de facto uses.

A mobile mapping system (see Guidelines Report page 34) is made up of a number of components that can include one or multiple scanners; camera(s); a computer and positioning sensors such as those that work with a global navigation satellite system, or GNSS; and an inertial measurement unit, or IMU. The cameras and the scanners are used to record the scene while the GNSS and IMU are determining the position and orientation of the data capture sensors. The latter determines the trajectory of the system. The trajectory is used to determine the three dimensional coordinates of each point collected by the scanner. Note that a mobile mapping system does not always include a laser scanner.

Stop-and-go refers to the use of a vehicle to move the scanner from position to position, making it mobile, but the vehicle does not move while the data is being collected. In stop and go there is no need for positioning sensors (such as the IMU or GNSS) as the location of the scanner can be determined from the use of ground control targets. This can lead to improved accuracy over true mobile scanning; however, will not usually be as efficient as true mobile scanning.

In order for the scanned data to be georeferenced in real world coordinates the mobile LIDAR scanning system relies on GPS, or more correctly a Global Navigation Satellite System – GNSS. By accurately time stamping and synching the navigation data, the trajectory of the mobile platform can be recorded. Physically surveyed ground control points can then be used to further improve the accuracy of the position for each 3D point captured by the scanner.

An IMU is an inertial measurement unit. In between GNSS updates and/or when the GNSS receiver is not able to obtain high quality position information the mobile LIDAR system can make use of the IMU data to determine the change in position and attitude since the last acceptable fix. This can occur when the system is inside a tunnel or urban canyon. The IMU uses a combination of accelerometers and gyroscopes to determine the velocity and orientation of the platform. Typically, the IMU can only be relied on for relatively short periods of time due to the internal drift in the system.

Spot size refers to the area of the laser light beam as it intersects a surface. Although laser light is tightly collimated the size of the spot increases with the distance from the scanner. In certain cases one portion of the spot may be reflected by an object while the remainder of the beam continues on its path until it is reflected or disappears. This is what results in multiple returns from a single laser beam.

Multiple returns occur when a portion of the laser beam is reflected by an object while the remainder continues on its path (see spot size). This can be seen in the Guidelines Report in Appendix A.3. Some systems can record up to 5 returns from a single pulse. This phenomenon can be seen in the “penetrating” of vegetation. This is one of the key benefits of airborne LIDAR over photogrammetry for forest mapping since all it takes is a portion of the active laser pulse to be returned from the ground to develop the ground contours.

Full waveform is the ability of a LIDAR sensor to record the entire shape of the return LIDAR pulse. This can allow for more detailed analysis of the objects being scanned and some believe identification of material types from the unique waveform “signature”. This is further explained in Appendix A.3 in the Guidelines Report.

There are a number of items that contribute to the error budget for mobile LIDAR. These include scanner errors, the GNSS positional accuracy, the quality of the IMU, lever arm offsets for each sensor, accuracy of the ground control and other factors including the expertise of the technicians. See Chapter 10 in the Guidelines Report for additional information.