Timmy Gambin

Archaeology via underwater robots: Mapping and localization within maltese cistern systems

This paper documents the application of several underwater robot mapping and localization techniques used during an archaeological expedition. The goal of this project was to explore and map ancient cisterns located on the islands of Malta and Gozo. The cisterns of interest acted as water storage systems for fortresses, private homes, and churches. They often consisted of several connected chambers, still containing water. A sonar-equipped Remotely Operated Vehicle (ROV) was deployed into these cisterns to obtain both video footage and sonar range measurements. Four different mapping and localization techniques were employed including 1) Sonar image mosaics using stationary sonar scans, and 2) Simultaneous Localization and Mapping (SLAM) while the vehicle was in motion, 3) SLAM using stationary sonar scans, and 4) Localization using previously created maps. Two dimensional maps of 6 different cisterns were successfully constructed. It is estimated that the cisterns were built as far back as 300 B.C.

Surface reconstruction of Maltese cisterns using ROV sonar data for archeological study

We present a methodology and algorithm for the reconstruction of three dimensional geometric models of ancient Maltese water storage systems, i.e. cisterns, from sonar data. This project was conducted as a part of a four week expedition on the islands of Malta and Gozo. During this expedition, investigators used underwater robot systems capable of mapping ancient underwater cisterns and tunnels. The mapping included probabilistic algorithms for constructing the maps of the sonar data and computer graphics for surface reconstruction and visualization. This paper presents the general methodology for the data acquisition and the novel application of algorithms from computer graphics for surface reconstruction to this new data setting. In addition to reconstructing the geometry of the cisterns, the visualization system includes methods to enhance the understanding of the data by visualizing water level and texture detail either through the application of real image data via projective textures or by more standard texture mapping techniques. The resulting surface reconstructions and visualizations can be used by archaeologists for educational purposes and to help understand the shape and history of such water receptacles.

Mapping and visualizing ancient water storage systems with an ROV — An approach based on fusing stationary scans within a particle filter

This paper presents a new method for constructing 2D maps of enclosed underwater structures using an underwater robot equipped with only a 2D scanning sonar, compass and depth sensor. In particular, no motion model or odometry is used. To accomplish this, a two step offline SLAM method is applied to a set of stationary sonar scans. In the first step, the change in position of the robot between each consecutive pair of stationary sonar scans is estimated using a particle filter. This set of pair wise relative scan positions is used to create an estimate of each scans position within a global coordinate frame using a weighted least squares fit that optimizes consistency between the relative positions of the entire set of scans. In the second step of the method, scans and their estimated positions act as inputs to a mapping algorithm that constructs 2D octree-based evidence grid maps of the site. This work is motivated by a multi-year archeological project that aims to construct maps of ancient water storage systems, i.e. cisterns, on the islands of Malta and Gozo. Cisterns, wells, and water galleries within fortresses, churches and homes operated as water storage systems as far back as 2000 B.C. Using a Remotely Operated Vehicle (ROV) these water storage systems located around the islands were explored while collecting video, still images, sonar, depth, and compass measurements. Data gathered from 3 different expeditions has produced maps of over 60 sites. Presented are results from applying the new mapping method to both a swimming pool of known size and to several of the previously unexplored water storage systems.

Ontology-Based Photogrammetric Survey in Underwater Archaeology

This work addresses the problem of underwater archaeological surveys from the point of view of knowledge. We propose an approach based on underwater photogrammetry guided by a representation of knowledge used, as structured by ontologies. Survey data feed into to ontologies and photogrammetry in order to produce graphical results. This paper focuses on the use of ontologies during the exploitation of 3D results. JAVA software dedicated to photogrammetry and archaeological survey has been mapped onto an OWL formalism. The use of procedural attachment in a dual representation (JAVA - OWL) of the involved concepts allows us to access computational facilities directly from OWL. As SWRL The use of rules illustrates very well such ‘double formalism’ as well as the use of computational capabilities of ‘rules logical expression’. We present an application that is able to read the ontology populated with a photogrammetric survey data. Once the ontology is read, it is possible to produce a 3D representation of the individuals and observing graphically the results of logical spatial queries on the ontology. This work is done on a very important underwater archaeological site in Malta named Xlendi, probably the most ancient shipwreck of the central Mediterranean Sea.

Cinematographic and Geometric Criteria for Virtual Camera Path Generation for the Visualization of Shipwreck Data

This paper presents the use of cinematographic and geometric measures for determining the path of a virtual camera for the generation of computer graphics video sequences focused on showing an underwater shipwreck. This work rises from the applied challenge of mapping underwater shipwrecks, reconstructing a computer graphics model then creating an educational visualization of the wreck. The primary algorithm presented in this work computes the optimal camera pitch and height along a path utilizing a probabilistic roadmap (PRM) that weights nodes using a computational models of cinematographic and geometric principles. These principles were used to evaluate potential viewpoints and influence whether or not a view is used in the final path. Specifically, the algorithm uses a computational evaluation of the cinematographic ‘rule of thirds’ and a geometric evaluation of the model normals relative to the camera viewpoint. A user study of video output from the system suggests that our computed paths are ranked higher than simple circular or random paths and that the ‘rule of thirds’ is a more important criteria than the geometric principle explored.

Submerged speleothem in Malta indicates tectonic stability throughout the Holocene

Submerged caves represent potential archives of speleothems with continental and marine biogenic layers. In turn, these can be used to reconstruct relative sea-level changes. This study presents new data on the tectonic behaviour of the island of Malta during the Holocene. These data were obtained from a speleothem sampled, during an underwater survey, at a depth of −14.5 m, inside a recently discovered submerged cave. Since the cave was mainly formed in a subaerial karst environment, the presence of a speleothem with serpulids growing on its continental layers permitted the reconstruction of the chronology for drowning of the cave. The radiocarbon dates obtained from the penultimate and last continental layers of the speleothem, before a serpulid encrustation, were compared with synthetic aperture radar (SAR) and global positioning system (GPS) data, together with published sedimentological and archaeological data. The radiocarbon analyses provided an average age of 7.6 ka BP that perfectly aligns with the Lambeck’s model of Holocene sea level. Morevoer, long-term data agree with published archeological and sedimentological data as well as with SAR interpherometric and GPS trends on a decadal scale. We conclude that the Maltese islands were tectonically stable during the Holocene, and this tectonic behaviour still persists nowadays. On the contrary, new informations on older deposits, such as MIS5e (Maritime Isotope Stage, corresponding to 125 ka ago) were not found in the study area, confirming the lack of older Quaternary marine deposits in these islands.

AUV motion-planning for photogrammetric reconstruction of marine archaeological sites

This paper presents a method for constructing 3D maps of marine archaeological sites using deployments of Autonomous Underwater Vehicles (AUV) equipped with sonar and cameras. The method requires multiple AUV missions in which the first mission directs the AUV to conduct a high altitude lawnmower scan over the area to create a course bathymetry map using sonar. Subsequent AUV missions then direct the AUV to make low altitude fly-overs just above the wreck with the goal of obtaining camera images from multiple viewpoints of the wreck to enable offboard 3D mapping via photogrammetric reconstruction. This approach uses a coarse map generated after the first mission to construct AUV paths that attempt to maximize information gain, i.e. maximize the number of viewpoints of the wreck within a time limit. Presented is a motion planner derived from Rapidly-Exploring Random Trees (RRT) that have sampling strategies modified for this problem. Specifically, the random node selection and new node generation are designed to consider the kinematics of an AUV and the information gain associated with each flyover. Simulation results demonstrate improvements of up to 152% when these sampling strategies are used. Experiment results, involving deployments for mapping two known wrecks located along the coast of Malta, validate the systems ability to construct 3D maps and associated visualizations.