DIRECT ESTIMATION OF THE RELATIVE ORIENTATION IN UNDERWATER ENVIRONMENT

While accuracy, detail, and limited time on site make photogrammetry a valuable means for underwater mapping, the establishment of reference control networks in such settings is oftentimes difficult. In that respect, the use of the coplanarity constraint becomes a valuable solution as it requires neither knowledge of object space coordinates nor setting a reference control network. Nonetheless, imaging in such domains is subjected to non-linear and depth-dependent distortions, which are caused by refractive media that alter the standard single viewpoint geometry. Accordingly, the coplanarity relation, as formulated for the in-air case does not hold in such environment and methods that have been proposed thus far for geometrical modeling of its effect require knowledge of object-space quantities. In this paper we propose a geometrically-driven approach which fulfills the coplanarity condition and thereby requires no knowledge of object space data. We also study a linear model for the establishment of this constraints. Clearly, a linear form requires neither first approximations nor iterative convergence scheme. Such an approach may prove useful not only for object space reconstruction but also as a preparatory step for application of bundle block adjustment and for outlier detection. All are key features in photogrammetric practices. Results show that no unique setup is needed for estimating the relative orientation parameters using the model and that high levels of accuracy can be achieved.

ASSESSMENT OF A LOW-COST SIDE-SCAN SONAR FOR RIVER ESTUARY UNDERWATER IMAGING

River and estuary areas commonly exhibit complex and heterogeneous habitats. Thus, revealing the distribution of riverbed morphologies could promote the area management and habitats protection. Since the remote sensing method and manual survey are limited to use, side-scan sonar performs an expectant outcome in underwater habitat imaging. In shallow water and stream areas, low-cost side-scan sonar imaging has become a notable subject of study, yet its use in Indonesia is still limited. This study describes an investigation of the use of a recreational-grade side-scan sonar for stream underwater imaging. A visual inspection and interpretation were implemented using a free-cost sonar software. The result shows some underwater objects and debris could be portrayed perfectly and this indicates that the inexpensive sonar system is appropriate to be used in shallow water and stream areas with a non-rough sea surface. It is suggested that this system could provide a satisfactory product to the users who do not require high accuracy and high resolution of riverbed imagery.Keywords: estuary, river, underwater mapping, acoustic remote sensing, low-cost, side-scan sonar.

On the Information Advantage of Sidescan Sonar Three-Frequency Colour over Greyscale Imagery

A prototype three-frequency (114, 256, and 410 kHz) colour sidescan sonar system, built by Kongsberg Underwater Mapping Ltd. (Great Yarmouth, UK), was previously described, and preliminary results presented, in Tamsett, McIlvenny, and Watts. The prototype system has subsequently been modified, and in 2017, new data were acquired in a resurvey of the Inner Sound of the Pentland Firth, North Scotland. An image texture characterisation and image classification exercise demonstrates considerably greater discrimination between different seabed classes in a three-frequency colour sonar image of the seabed, than in a multi-frequency colour image reduced to greyscale display, or in a single-frequency greyscale image, with readily twice the number of classes of seabed discriminated between, in the colour image. The information advantage of colour acoustic imagery over greyscale acoustic imagery is analogous to the information advantage of colour television images over black-and-white television images. A three-frequency colour sonar image contains a theoretical maximum of a factor of 3 times the information in a corresponding greyscale image, for independent seabed responses at the three frequencies. Estimates of the average information per pixel (information entropy) in the colour image, and in corresponding greyscale images, reveal an actual information advantage of colour sonar imagery over greyscale, to be in practice approximately a factor of 2.5, empirically confirming the greater information based utility of three-frequency colour sonar over greyscale sonar. Reference: Tamsett, D.; McIlvenny, J.; Watts, A. J. Mar. Sci. Eng. 2016, 4(26).

PRELIMINARY EXPERIMENTAL STUDY ON DESIGNING BALLAST SYSTEM FOR AUTONOMOUS UNDERWATER VEHICLE

ABSTRACT Unmanned submarine commonly called Autonomous Underwater Vehicle (AUV) is one type of underwater robots used for underwater mapping. AUV is an underwater vehicle capable of automatically moving in water, controlled by humans on vessel. To build AUV is not easy as many components play important roles in the operation of AUV, one of which is active ballast. Calculations on the making and benchmarks of active ballast systems are still very rare. Active ballast is a set of equipment used to fill its tanks with seawater and to empty sea water from the tanks on submarines. These tanks are intended to balance submarines and for active ballast systems on submarines so as to be able to dive and float as needed. In this paper an experimental study was carried out on a tube resembling AUV with both fresh water and sea water to obtain ballast volume in AUV. Keywords: AUV, Active ballast, Experimental study

A Novel Algorithm for Quasi Real-Time Matching of Bathymetric Data

Matching two regions represented in bathymetric data that have some form of geographical overlapping is an important and challenging aspect in underwater mapping. It is important because of the possible error in estimating the geographical location of each point underwater. It is challenging due to the size of the acquired bathymetric data points. The matching could also play a vital role in the registration of underwater images and/or maps fusion, if both bathymetric and intensity scans are considered. Compared to the exhaustive search that requires polynomial time, O(n2), an efficient bathymetric matching algorithm is proposed in this work that finds several match-points in linear time, requiring thus O(n) computations. The paper thus presents a new algorithm that allows to compile the bathymetric data of the common areas of two submarine areas that have been sampled in underwater missions.