A novel Structure from Motion-based approach to underwater pile field documentation

Abstract

This article presents a novel methodology to the underwater documentation of pile fields in archaeological lakeside settlement sites using Structure from Motion (SfM). Mapping the piles of such sites is an indispensable basis to the exploitation of the high resolution absolute chronological data gained through dendrochronology. In a case study at the underwater site of Ploča, Mičov Grad at Lake Ohrid, North Macedonia, nine consecutive 10 m 2 strips and a 6 m 2 excavation section were uncovered, the situation documented, and the wood piles sampled. The gained data was vectorized in a geographic information system. During two field campaigns, a total of 794 wooden elements on a surface of 96 m 2 could be documented three-dimensionally with a residual error of less than 2 cm. The exceptionally high number of fishes in the 5 m deep water resulted in a significant covering of potentially important information on the relevant photos. We present a machine learning approach, especially developed and successfully applied to the automatic detection and masking of these fishes in order to eliminate them from the images. The discussed documentation workflow enables an efficient, cost-effective, accurate and reproducible mapping of pile fields. So far, no other method applied to the recording of pile fields has allowed for a comparably high resolution of spatial information.

Delivering the Deep: Maritime archaeology for the 21st century: selected papers from IKUWA 7

“…Taking precise global coordinates on an underwater site has always been a challenge, especially in deep water, because the signal from the satellites cannot penetrate through the water's surface. Fortunately, the Ropotamo site is at a relatively shallow depth and can be measured by just using a 5 m-long pole to keep the transceiver of the signal above water, a method which has been widely used in recent years (Pacheco-Ruiz et al 2018: 125;Reich et al 2021). Coupled with the millimetric intra-site accuracy noted above, this fixes the position of the trench and every object in the model down to 2 cm of global accuracy.…”

High-accuracy height differences using a pressure sensor for ground control points measurement in underwater photogrammetry

“…In very shallow waters (< 5 m), the use of high-precision differential GNSS measurements such as RTK or PPK methods was tested using a special prototype consisting of a buoy with an extensible rod connected to a tilt sensor (Reich et al, 2021). A more classic rod kept vertical in Balletti et al (2015) or a 2 m high tripod (Wright et al, 2020) were also employed.…”

Evaluation of the Accuracy of Photogrammetric Reconstruction of Bathymetry Using Differential GNSS Synchronized with an Underwater Camera

“…Direct measurement of control point positions has also been demonstrated, using geodetic equipment adapted for underwater operations such as a tripod and multiple meters of extension poles to elevate a GNSS receiver above the surface of the water, but this technique is limited to shallow areas (Balletti et al, 2015, Wright, 2020. A similar concept for shallow water was demonstrated in Reich et al (2021) using a prototype RTK buoy with an extensible rod for measuring ground control points underwater. Compared to GNSS receiver usage in terrestrial settings, the underwater setup is more cumbersome, and accuracy is reduced by potential deflection of the longer pole as well as induced motion from wave action and currents.…”