The Range and Intensity Calibration of the Mobile Laser Scanner using Portable Calibration Targets
Mobile Laser Scanning, also called Mobile Terrestrial Laser Scanning, (MLS) is currently a rapidly developing area in LS where laser scanners, GNSS and IMU are mounted onboard a moving vehicle. MLS can be considered to fill the gap between ALS and TLS. In MLS, the data collection can be performed either in so-called stop-and-go mode or in continuous mode. The stop-and-go mode corresponds to conventional TLS measurements and therefore MLS, hereafter, refers to the continuous model, i.e. the use of continuous scanning measurements along the drive track. In addition to laser scanners, the data acquisition sensors can include, e.g., digital cameras or video cameras.
Data provided by MLS systems can be characterized with the following technical parameters: a) point density in the range of 100-1000 pulses per m2 at the 10 m distance, b) distance accuracy of 2-5 cm, and c) operational scanning range from 1 to 100 m.
Currently, there is an increasing number of research systems e.g., Geomobil (ICC), GeoMaster (University of Tokyo), Lara-3D (Ecoles des Mines de Paris), ROAMER and Sensei (FGI), and commercial and custom-made systems (for example Optech Lynx mobile mapper, Streetmappers of 3D Laser Mapping based on Riegl scanners, Mitsubishi using Sick LMS 291, Riegl VMX-250 integrating two Riegl VQ-250 scanners, Topcon-made systems to Google IP-S2 having three Sick LMS 291 scanners, Trimble Trident-3D based on Sick and Riegl scanners). The amount of data produced by such systems is huge (at the rate of 0.25-1 M pts/s), and manual processing of the data is very time-consuming, which prompts a need for automated methods that decrease the amount of manual work required to produce accurate 3D models. At present, it is possible to use software and methods developed for terrestrial and airborne laser scanning, but due to different scanning geometry, different point density and the fast processing needed, algorithms for MLS data processing need also to be developed separately.
Mobile laser scanners are widely used for mapping purposes in urban areas, factory environments, historical sites and monuments, indoors and in forestry to name some applications. Scanners are factory calibrated before delivering to the customer, because the distance measurements depend on the object brightness in a way that bright objects appear to be closer to the laser scanner than the dark ones. However, the practice has shown that the error in question may still remain in the field measurements. Also the intensity value should be corrected. In summary, there are many areas where calibration of mobile laser scanning is needed (localization, system calibration e.g. between different scanners of the same system, point cloud matching, intensity calibration).
Objective of this paper is to show that by using portable calibration targets of various reflectance values, e.g. of the size of 1-m-by-1-m, both the range affecting the LUT and intensity affecting the usability of the intensity data can be performed simultaneously. Both range and reflectance properties were measured with an infrared camera and a laser scanner both indoors and in the street environment to confirm the calibration.
The results show that these portable calibration targets improve the usability of the both range and intensity information in mobile laser scanning.
Keywords: Mobile mapping, Mobile laser scanning, Calibration, Laser scanning, Accuracy, LUT, Radiometric calibration of Intensity