## 2 Benchmarking Mobile Laser Scanning Systems
### Materials
#### 2.1.2 Reference Data for Accuracy Evaluation  
Dense terrestrial laser scanner point clouds were used to obtain the reference targets for the analysis of geometric accuracy. The reference point clouds were collected on 7 May 2009 using FGI's mobile mapping system called the ROAMER [11], Road Environment Mapper in static mode. The ROAMER was installed on the roof of a car, and the car was kept standing static on the road during each 360\({}^{\circ}\) scanning performed using of the FARO Photon 80 terrestrial scanner (Figure 3). The scan resolution was set to 0.0013 rad point separation. The georeferencing of individual scannings was computed during post-processing: the scanner position and heading were obtained from the ROAMER's SPAN navigation system and the scannings were leveled using the scanner's built-in inclinometer. The virtual GPS reference station data used in GPS post-processing were downloaded from the GPSNet.fi service. The SPAN data were processed using the Waypoint Inertial Explorer software, which gave the estimated accuracies of 11 mm in 3D-position and 0.027\({}^{\circ}\) in heading (RMS) for the ROAMER's inertial measurement unit (IMU) during the measurements. The offset between the IMU and the scanner origin, as well as the offset between the SPAN and the scanner heading, were determined during system calibration. The ROAMER's TLS/static data were validated against 150 check points measured using a total station (Trimble 5602S DR200+). A total of nine ground control points (GCPs) were measured for the total station setup around the Espoonlahti test field using repeated real-time GPS measurements (Leica SR530). Eight individual measurements were taken at each point using different reference data sources (RTK-GPS using its own reference station and VRS-GPS using a virtual reference station) and different satellite constellations (a few hours passing between the measurement sessions). For each GCP, the first two sessions were measured using RTK-GPS (with an expected accuracy of 1 cm + 1-2 ppm in horizontal plane and 1.5-2 cm + 2 ppm in height [30]), and then two sessions using VRS-GPS (with an expected accuracy of 2 cm in horizontal plane and 4 cm in height [31]). A new GPS initialization was acquired between each session. This procedure was carried out for a second time after a few hours. The GCP coordinates were then computed as a mean of the obtained eight coordinates. The maximum  
Figure 3: An example of reference point clouds. The scanning locations can be seen as shadows on the road that by the car was driven along. standard deviation of the eight \"original\" coordinates was 23 mm in horizontal plane and 30 mm in elevation (the averages were 13 mm and 20 mm, respectively). With most of the ROAMER scannings, the comparisons showed that the check points and point clouds matched one another within the standard deviation of the GCPs, _i.e.,_ a couple of centimeters, but with a few scans there was error in the leveling of the point cloud. In these cases there had been a passing bus visible in the scanning data, and so it is obvious that a large vehicle had caused a disturbance in the functioning of the scanner's inclinometer. These point clouds were then re-leveled by using neighboring point clouds and check points. Following the point cloud validation, the targets for accuracy analysis were measured along a 350 m length of the test field's Section A (Figure 1) with the best GNSS visibility. The TerraScan-software by TerraSolid Ltd. was used for all point cloud operations. Firstly, the ground points were classified and a regular grid with a point spacing of 5 cm was computed to achieve an even distribution of the ground points. This grid was then thinned by selecting every 1,000 th point, and these thinned points were compared to the original ground points.