## 2 Benchmarking Mobile Laser Scanning Systems
### Materials
#### Benchmarked Mobile Laser Scanning Data
The laboratory calibration was based on measuring the physical offsets and rotations between the scanner, IMU and the GPS antenna. This laboratory calibration was fine-tuned using the measured data, e.g., by utilizing the data acquired by driving the same location in two directions and using some control targets. These fine-tuned calibration values were applied in recomputing the data, and in producing the second set of point clouds for analysis. Figure 9: Sensei data. Due to the scanning angle, the Sensei covered only one side of the trajectory at a time. The RIEGL VMX-250 (Figure 11) was introduced at the beginning of 2010 and the test field data were acquired in March 2010. The system consists of two RIEGL VQ-250 scanners and a navigation unit with IMU, GNSS and odometer instruments. Each of the scanners measures up to 300,000 points and 100 profiles per second. The maximum measurement range is 500 m. In September 2011, RIEGL announced another mobile mapping system, the VMX-450, with VQ-450 scanners, and the capability to measure up to 550,000 points and 200 profiles per second. RIEGL delivered two point clouds for analysis. The first batch of data was received in June 2010. Later on, RIEGL announced that they have developed their system calibration further and wish to implement their latest expertise also in the test field data. A small set of control points from the test field was delivered to RIEGL to assist in the calibration procedure. The second batch of data was received in May 2011. Figure 10: The ROAMER. The FGI Sensei is a low-cost modular measurement system consisting of a number of measurement instruments. These include a GPS/IMU positioning system, two laser scanners, a CCD camera, a spectrometer and a thermal camera. The above GPS/IMU system is a NovAtel SPAN-CPT integrated GPS/INS receiver system, embodying NovAtel's OEMV GNSS precision receiver technology with three fiber optic gyros and three MEMS accelerometers in a single unit. The SPAN-CPT delivers 3D position, velocity and attitude solutions. The measurements of the different INS subsystems are combined using Waypoint Inertial Explorer and GPS virtual reference station (VRS) data. An Ibeo Lux laser scanner was used on the Espoonlahti test field. The Ibeo Lux simultaneously measures points from four different layers and it is theoretically capable of measuring up to 38,000 points/second if only one return per pulse per layer is assumed. The scanner is able to record up to three returns per pulse per layer, thus enabling it to get hits from building walls or the ground even when these are covered by nearby trees or other vegetation. Its distance measurement range is 0.3 m to 200 m (50 m for targets with 10% remission), its ranging accuracy is 10 cm, its angular resolution is 0.25\({}^{\circ}\) and the divergence of its laser beam is 1.4 mrad horizontally and 14 mrad vertically with respect to the scanner body, meaning that objects may appear elongated in the vertical direction. When mounted on the roof of a car (Figure 12), this vertical elongation turns into a horizontal error as the scanner is mounted vertically. For this reason, most of the reference targets for analysis of planimetric accuracy could not be reliably measured, and the performance analysis was completed only for elevation accuracy. When the Sensei is used on the roof of a car, the instruments point towards the side of the car, and the laser scanner scans vertically, and thus covers only one side of the trajectory at a time [39]. Figure 11: The RIEGL VMX-250 [38]. The Streetmapper 360 (Figure 13) was launched in October 2008 and test site data were acquired in June 2011. The system consists of two RIEGL VQ-250 scanners and a navigation unit with a fiber-optic gyro-based IMU, GPS and Direct Inertial Aiding (DIA) to assist in areas of poor GPS reception. Each scanner measures up to 300,000 points and 100 profiles per second. The maximum measurement range is 500 m.  
Figure 12: The Sensei mounted on the roof of a car.