COMPARISON of DIVER-OPERATED UNDERWATER PHOTOGRAMMETRIC SYSTEMS for CORAL REEF MONITORING

Underwater photogrammetry is a well-established technique for measuring and modelling the subaquatic environment in fields ranging from archaeology to marine ecology. While for simple tasks the acquisition and processing of images have become straightforward, applications requiring relative accuracy better then 1:1000 are still considered challenging. This study focuses on the metric evaluation of different off-the-shelf camera systems for making high resolution and high accuracy measurements of coral reefs monitoring through time, where the variations to be measured are in the range of a few centimeters per year. High quality and low-cost systems (reflex and mirrorless vs action cameras, i.e. GoPro) with multiple lenses (prime and zoom), different fields of views (from fisheye to moderate wide angle), pressure housing materials and lens ports (dome and flat) are compared. Tests are repeated at different camera to object distances to investigate distance dependent induced errors and assess the accuracy of the photogrammetrically derived models. An extensive statistical analysis of the different systems is performed and comparisons against reference control point measured through a high precision underwater geodetic network are reported

IMAGE PRE-PROCESSING STRATEGIES FOR ENHANCING PHOTOGRAMMETRIC 3D RECONSTRUCTION OF UNDERWATER SHIPWRECK DATASETS

Although underwater photogrammetry has become widely adopted, there are still significant unresolved issues that are worthy of attention. This article focuses on the 3D model generation of underwater shipwrecks and intends explicitly to address the problem of dealing with sub-optimal datasets. Even if the definition of best practices and standards to be adopted during the acquisition phase appears to be crucial, there is a massive amount of data gathered so far by professionals and the scientific community all over the world that cannot be ignored. The compelling idea is to attempt to achieve the best reconstruction results possible, even from sub-optimal or less-than-ideal image datasets. This work focuses on the investigation of different strategies and approaches for balancing the quality of the photogrammetric products, without neglecting their reliability concerning the surveyed object. The case study of this research is the Mandalay MHT, a 34 m long steel-hulled auxiliary schooner that sank in 1966 and now lies in the Biscayne National Park (Florida - USA). The dataset has been provided by the Submerged Resources Center (SRC) of the US National Park Service, in order to develop an experimental image enhancement method functional to the virtualization and visualization of the generated products, as a part of a sustainable, affordable, and reliable method of studying submerged artefacts and sites. The original images have been processed using different image enhancement approaches, and the outputs have been compared and analysed

VIRTUAL TOUR IN THE SUNKEN “VILLA CON INGRESSO A PROTIRO” WITHIN THE UNDERWATER ARCHAEOLOGICAL PARK OF BAIAE

The paper presents the application of some Virtual Reality technologies developed in the Horizon 2020 i-MARECulture project to the case study of the sunken “Villa con ingresso a protiro”, dated around the II century AD, and located in the Marine Protected Area - Underwater Park of Baiae (Naples).The i-MARECulture project (www.imareculture.eu), in fact, aims to improve the public awareness about the underwater cultural heritage by developing new tool and techniques that take advantage of the virtual reality technologies to allow the general public to explore the archaeological remains outside of the submerged environment.To this end, the paper details the techniques and methods adopted for the development of an immersive virtual tour that allow users to explore, through a storytelling experience, a virtual replica and a 3D hypothetical reconstruction of the complex of the “Villa con ingresso a protiro”.</p

COMPARISON OF DIVER-OPERATED UNDERWATER PHOTOGRAMMETRIC SYSTEMS FOR CORAL REEF MONITORING

Underwater photogrammetry is a well-established technique for measuring and modelling the subaquatic environment in fields ranging from archaeology to marine ecology. While for simple tasks the acquisition and processing of images have become straightforward, applications requiring relative accuracy better then 1:1000 are still considered challenging. This study focuses on the metric evaluation of different off-the-shelf camera systems for making high resolution and high accuracy measurements of coral reefs monitoring through time, where the variations to be measured are in the range of a few centimeters per year. High quality and low-cost systems (reflex and mirrorless vs action cameras, i.e. GoPro) with multiple lenses (prime and zoom), different fields of views (from fisheye to moderate wide angle), pressure housing materials and lens ports (dome and flat) are compared. Tests are repeated at different camera to object distances to investigate distance dependent induced errors and assess the accuracy of the photogrammetrically derived models. An extensive statistical analysis of the different systems is performed and comparisons against reference control point measured through a high precision underwater geodetic network are reported

Guidelines for Underwater Image Enhancement Based on Benchmarking of Different Methods

Images obtained in an underwater environment are often affected by colour casting and suffer from poor visibility and lack of contrast. In the literature, there are many enhancement algorithms that improve different aspects of the underwater imagery. Each paper, when presenting a new algorithm or method, usually compares the proposed technique with some alternatives present in the current state of the art. There are no studies on the reliability of benchmarking methods, as the comparisons are based on various subjective and objective metrics. This paper would pave the way towards the definition of an effective methodology for the performance evaluation of the underwater image enhancement techniques. Moreover, this work could orientate the underwater community towards choosing which method can lead to the best results for a given task in different underwater conditions. In particular, we selected five well-known methods from the state of the art and used them to enhance a dataset of images produced in various underwater sites with different conditions of depth, turbidity, and lighting. These enhanced images were evaluated by means of three different approaches: objective metrics often adopted in the related literature, a panel of experts in the underwater field, and an evaluation based on the results of 3D reconstructions