Comparison between the Employment of a Multibeam Echosounder on an Unmanned Surface Vehicle and Traditional Photogrammetry as Techniques for Documentation and Monitoring of Shallow-Water Cultural Heritage Sites: A Case Study in the Bay of Algeciras

Abstract

Over the last few years, due to various climatic, anthropogenic, and environmental factors, a large amount of submerged heritage has been unearthed and exposed to deterioration processes in the Bay of Algeciras. These impacts can be more severe in shallow waters, where the cultural heritage is more vulnerable to natural and human-induced impacts. This makes it urgent to document cultural heritage at risk of disappearing using different techniques whose efficiencies in the archaeological record need to be determined and compared. For this purpose, we have documented a shipwreck in the Bay of Algeciras using two techniques: photogrammetry and a multibeam echosounder. The photogrammetric method consists of obtaining a 3D model from numerous photographs taken of an object or a site. The processing software creates three-dimensional points from two-dimensional points found in the photographs that are equivalent to each other. Multibeam echosounders are capable of providing side scan imagery information in addition to generating contour maps and 3D perspectives of the surveyed area and can be installed in an unmanned surface vehicle. As a result, we have obtained two 3D visualisations of the shipwreck, i.e., digital copies, that are being used both for the analysis of its naval architecture and for its dissemination. Through the comparison of the two techniques, we have concluded that while a multibeam echosounder provides a detailed digital terrain model of the seabed, photogrammetry performed by divers gives the highest resolution data on objects and structures. In conclusion, our results demonstrate the benefits of this combined approach for accurately documenting and monitoring shipwrecks in shallow waters, providing valuable information for conservation and management efforts.