DOI: 10.23784/HN116-03
Underwater photogrammetry
HN 116 — 06/2020
23
Fusing ROV-based photogrammetric
underwater imagery with multibeam
soundings for reconstructing
wrecks in turbid waters
An article by ROBIN ROFALLSKI, PATRICK WESTFELD, JEAN-GUY NISTAD, ANNETT BÜTTNER and THOMAS LUHMANN
Observation and monitoring ofwrecksarean integral part of the duties of hydrographic
offices such as BSH. A common practice consists of first surveying wrecks using vessel-
based multibeam echo sounding systems and subsequently having divers visually in
spect them. In order to provide an objective procedure and seta baseline for monitoring
wrecks, unmanned underwater vehicles equipped with imaging systems can be used to
inspect wrecks and other obstructions in more details. This paper presents a workflow
for combining multibeam soundings and photogrammetric point clouds generated by a
ROV-based camera system. Structure from motion and image enhancement are used to
obtain a colour-coded point cloud, which is then fused and scaled with the multibeam
soundings, resulting in data densification on wrecks. Finally, the feasibility of integrating
this fused data to common hydrographic practice is demonstrated.
ROV | underwater photogrammetry | multibeam echo sounder | point cloud fusion
ROV | Unter-Wasser-Photogrammetrie | Fächerecholot | Punktwolkenfusion
Wracks zu suchen und zu überwachen gehört zu den Aufgaben von Hydrographischen Diensten wie
dem BSH. In der Praxis Ist es gängig, Wracks zunächst mit schiffsgestützten Fächerecholotsystemen zu
vermessen und anschließend von Tauchern visuell Inspizieren zu lassen. Um ein objektives Verfahren be
reitzustellen und eine Ausgangsbasis für die Überwachung von Wracks zu schaffen, können unbemann
te, mit blldgebenden Systemen ausgerüstete Unter-Wasser-Fahrzeuge eingesetzt werden, mit denen
Wracks und andere Hindernisse genauer Inspiziert werden. Dieser Beitrag stellt einen Arbeltsablauf zur
Kombination von Fächerecholotpellungen und photogrammetrischen Punktwolken vor, die von einem
ROV-baslerten Kamerasystem erzeugt werden. Strukturen aus Bewegung und Bildverbesserung werden
verwendet, um eine farbcodlerte Punktwolke zu erhalten, die dann mit den Fächerecholotpellungen ver
schmolzen und skaliert wird, was zu einer Datenverdichtung bei Wracks führt. Schließlich wird gezeigt,
dass die Integration dieser verschmolzenen Daten In der hydrographischen Praxis machbar Ist.
i Motivation and state of the art
Mapping underwater obstructions (e.g. wrecks,
rock fields) Is a crucial mandate for the German
Federal Maritime and Hydrographic Agency (Bun
desamt für Seeschifffahrt und Hydrographie, BSH)
and other Institutions Involved In safety of naviga
tion around the world. BSH alone Is responsible for
monitoring more than 2,500 underwater obstruc
tions In German territorial waters, regarded as po
tential hazards to shipping and fisheries (BSH 2020).
BSH presently carries out surveys of wrecks In
a two-stage process. Hydrographic vessels first
survey the wrecks using multibeam echo sound
ing systems (MBES) to obtain a georeferenced 3D
point cloud, I.e. sounding set. This data set pro
vides essential preliminary Information (e.g. posi
tion, shape, height above seafloor) to professional
divers, who subsequently Inspect the wrecks with
the aim of providing an accurate portrayal of
their state. This process Involves a detailed visual
and tactile Inspection as well as an Independent
measurement of the wrecks' shoalest point using
a pneumatic depth sensing hose pipe. Remotely
operated vehicles (ROV) equipped with camera
systems sometimes supplement or partially re
place the work of divers. Any potential change
(e.g. collapse, sedimentation, drift) from the pre
vious known state of a wreck Is subsequently re
ported.
Vessel-mounted MBES are widely established
for collecting bathymetry and detecting objects
underwater (Brisette et al. 1997). Wide swaths and
numerous narrow beams allow for efficient object
detection and Identification, even In turbid envl-
Authors
Robin Rofallski is a research
assistant at Jade University in
Oldenburg.
Dr. Patrick Westfeld, Jean-Guy
Nistad and Annett Buttner are
employed at BSH in Rostock.
Prof. Dr. Thomas Luhmann
teaches at Jade University in
Oldenburg.
robin.rofallski@jade-hs.de
