Assessing Mobile Mapping System Performance Conor Cahalane

1. Assessing Mobile Mapping System Performance Conor Cahalane Mobile Mapping Mobile Mapping Systems (MMS) enable high density spatial data to be collected along route networks and in urban environments. These data can then be utilised in a number of ways, such as route safety audits, road authority mapping, infrastructure surveys and change detection for national mapping agencies. Combining high accuracy navigation sensors, LiDAR (Figure 1 a) and imaging sensors onboard a moving platform (Figure 1 b) enable surveys to be carried out rapidly. Land based MMS compliment existing ground based and aerial surveying activities in a number of ways. Large scale information such as road sign detail or infrastructure condition can be recorded. Additionally, extensive ground control is not required and these systems can capture features that are sometimes obscured from aerial platforms. This project aims to assess the performance of MMS in terms of resolution, accuracy and repeatability for data capture. (a) (b) Figure 1: (a) LiDAR point cloud (b) StratAG’s XP1 Resolution Resolution - When a laser scanner is mounted on a moving platform, it is capable of producing millions of geo-referenced points which can then be used to create near-3D models. The development of processing algorithms for these point clouds has largely been the focus of the research community to date. However, given an arbitrary known static object positioned at a specific distance away from the MMS the resolution of the resulting point cloud that will describe that object is unknown. This is the underlying limit of all point cloud processing algorithms. We are in the process of developing a method for determining the quantitative resolution (in terms of point and profile spacing – Figure 2) of point clouds collected by a MMS with respect to known objects at specified distances. (a) (b) Figure 2: (a) Profile spacing (b) Point Spacing Accuracy Accuracy - The highly complimentary combination of GPS, an Inertial Navigation System (INS) and a Distance Measuring Instrument (DMI) give the XP1 the ability to directly georeference LiDAR points and imagery once all components are combined in the same coordinate system (Figure 3 a). The GPS provides high accuracy positional information but at a low frequency. The INS provides navigation updates at a higher frequency and when combined with the DMI is also capable of bridging any short GPS outages caused by trees, tunnels or urban canyons. The navigation solution is the largest source of error in a MMS (Glennie, 2007), and its effect upon point accuracies will be assessed by comparing against known targets or features (Figure 3 b). (a) (b) Figure 3: (a) Lever arms (b) Accuracy check – line is surveyed road marking overlayed on LiDAR pointcloud Repeatability (a) (b) Repeatability - For a MMS to be used for regular updating of route corridor mapping or for change detection it must be capable of producing data of equal quality on each occasion. However, as these systems rely heavily on GPS to directly georeference LiDAR points and imagery, a survey commissioned two weeks later than the original may be faced with very different GPS conditions (Barber et al., 2008). This effect (Figure 4 a) may be diminished by assessing the GPS quality pre-mission (Figure 4 b). The team at the NCG hope to quantify the effect that this error may have on the georeferenced point cloud.. Figure 4: (a) Multiple pass discrepancy – green first pass, blue second pass (b) GPS mission planning • References • Barber, D., Mills, J. & Smith-Voysey, S. [2008]. Geometric validation of a ground-based mobile laser scanning system . ISPRS Journal of Photogrammetry and Remote Sensing 63(1, January 2008), pp.128-141 . • Glennie, C. (2007). Rigorous 3D error analysis of kinematic scanning LIDAR systems. Journal of Applied Geodesy, 1, (January 2007) ,pp. 147-157. Research presented in this poster was funded IRCSET, Pavement Management Services and by a Strategic Research Cluster Grant (07/SRC/I1168) by Science Foundation Ireland under the National Development Plan. The author gratefully acknowledges this support.