T. Thies: A Vessel-Based Mobile Mapping System - From Sensor Integration to Multipurpose Products 28.02.2011
A Master Thesis submitted to HCU Hamburg for the degree of Master of Science, presented by Dipl.-Ing. (FH) Thomas Thies
For several years terrestrial laser scanners were used in multiple geodetic survey applications. These electro-optical measurement sensors which are based on further developments of LiDAR systems and total stations provide high accurate area based and contact-free object detection by using laser devices. For this reason laser scanner as relatively young mapping sensors have a solid position on the surveying market. Because of the rising demand on area based high resolution geo-data in the public and private sector whereas resolution means geometric but also time resolution, mobile mapping systems have been developed to combine the advantages of high speed terrestrial laser scanning and accurate positioning in kinematic and therefore also economic survey applications. At the moment these land vehicle mobile mapping systems were mainly used for road or railway track monitoring or inventory surveys and 3D city models. In the hydrographic branch vessel based mobile mapping systems which usually consist of a SONAR sensor, a positioning device and attitude and heading sensors were used for several decades. Multibeam echosounders provides area based depth information of the seabed by using hydro-acoustic measurement methods. The integration of hydrographic mapping sensors, positioning and attitude devices on one survey platform enables the kinematic determination of the seabed topography. Since hydrographic surveys usually are bordered by the coast line because of the measurement principle, the combination of laser scanning and echosounding provides seamless transition between “wet” and “dry” datasets.
The main objective of this thesis is even the integration of different mapping sensors to set up a vessel based mobile mapping system which fulfils the requirements of multiple survey applications in the Port of Hamburg. The focus of the work is split into three key aspects, the configuration of a mobile mapping system containing laser scanning and echosounding sensors, an accuracy analysis for the established system and practical test projects where the performance of the system in real-life conditions is evaluated.
The configuration of the multisensor system covers the sensor installation, integration and interfacing in the sensor network of the survey vessel whereas the main focus is on the spatial and temporal relationship between the systems. The determination of lever arms and angular misalignments between the sensors but also the time synchronization of all systems are mandatory to set up such a system.
The second part of the work is defined by an accuracy analysis of the established mobile system. In this context a a priori uncertainty estimation shows the possible performance of the survey system with respect to the individual sensor accuracies summarized by using the law of error propagation. A comparison of mobile measured datasets with a reference dataset determines the achieved absolute accuracy.
The final part of this investigation covers several performed survey projects in the Port of Hamburg which were done on the one hand to demonstrate potential port related applications for such a mapping system and on the other hand to present workflows to process recorded datasets in order to produce multipurpose products.