Digital monitoring, non-invasive recording, and 3D modeling of the submerged sites are emerging as fast growing fields within Underwater Archaeology and Historic Preservation. When time and funding are limited, Structure from Motion (SfM), the process by which two dimensional digital images are turned into a three dimensional digital model, can provide new angle for archaeological research.
During the era when roads were treacherous and far in between, water transport was common on lakes and rivers of the West, and Lake Tahoe, California was not an exception. Hidden under the surface of breathtaking Emerald Bay, which often is described as one of the natural wonders of the world, are two ordinary barges testifying to this maritime tradition. Built at the turn of the 19th and 20th centuries, their construction was simple. Yet, these water crafts were built with quality local material and attention to details. They were high functional, towed or rowed, and adapted to fulfill an important commercial need prior to the advent of rail and tracks. These barges and their subsequent demise reflected the general lifestyle and local ingenuity of the hardworking people who lived around the Lake Tahoe at the beginning of last century.
Although the excavation and recording of this site was completed in 1989-1990, these two submerged barges require continuous attention and monitoring. Located along the south-west shoreline of the Lake Tahoe inside the Emerald Bay, the barges are of a considerable archaeological, historical, and recreational significance. They are also part of the interpreted shipwreck within California State Parks system. As such, the goal of this 2014 survey (June 13 - 22) was to conduct a non-disturbance assessment of the site as part of the cultural monitoring and testing of the digital technologies for historic preservation. The project was a collaboration of AMARI, and the Center for Interdisciplinary Science of Art, Architecture and Archaeology (CISA3) at University of California, San Diego, and California State Parks.
The Underwater Site
Being positioned perpendicular to one another along the south-eastern shore in 5 to 40 ft of water, the site consists of two barges. Barge I, which originally protruded from water, is smaller and significantly more deteriorated and disarticulated of the two. It measures about 85 ft in length, 23 ft in breadth, and 44.5 inches in depth of hull. Barge II appears to be relatively intact. It measures 106 ft in length, 30 ft in breadth, and about 55 inches in depth. The construction technique behind those water crafts is simple as they resemble floating rectangular boxes with no superstructure. The barges were likely towed or rowed. They were constructed of local pine of size and type which reveal access to appropriate size timbers. At the same time, the use of nails, bolts, and corner plates reflects access to sufficient supply of iron. In short, the Emerald Bay barges represent general lifestyle and needs of the people who lived around the Lake Tahoe at the turn of 19th and 20th century (Smith, 1991).
Emerald Bay, Lake Tahoe, California
Digital Recording – Structure from Motion (SfM)
by Perry Naughton
Structure from motion, the process of turning two dimensional digital images into a three dimensional digital model, is emerging as a tool for archaeological research. At its most basic level, the structure from motion methodology only requires adequate coverage of sites and artifacts with digital photography. This provides many benefits: data collection can be performed rapidly, large amounts of digital data can be recorded and backed up in small amounts of physical space, dissemination of digital data is relatively easy, and the required tools (a digital camera) are inexpensive. The continued decrease in computing costs coupled with improved methods to create consistent, large scale, three dimensional structures will allow structure from motion to be used as a survey tool to both generate site maps and provide digital representation of artifacts that will allow measurements to be performed off site.
This stereo underwater camera rig was designed and built by Perry Naughton and Antonella Wilby from Center for Interdisciplinary Science of Art, Architecture and Archaeology (CISA3), Qualcomm Institute, University of California, San Diego. It was field tested on the Barge site at Emerald Bay during the 2014 season. The rig is designed to make high-accuracy models of underwater environments by fusing Structure from Motion (from) models from stereo cameras with measurements from a variety of other on-board sensors. Continue Reading…
There has been very little evaluation of how structure from motion methodologies perform for maritime archaeology. Fundamentally, structure from motion is built on an understanding of how light travels through air. Thus current software is limited in its inability to compensate for the bending of light rays as they travel through the water and camera enclosure interface. Additionally, underwater photography is an art form in itself. Backscattering and poor lighting limit the fidelity of three dimensional models of underwater structures.
The research goals of Emerald Bay Project addressed some of these challenges. We performed a quantitative evaluation of the degradation in accuracy resulting from collecting images underwater versus in air and currently work on building algorithms to compensate for this effect if necessary. We developed a camera rig to collect digital images along with odometry metadata from gyroscopes and accelerometers in order to increase the accuracy of the resulting models. The ultimate outcome of this project is to develop and evaluate a methodology to map and accurate measure shipwrecks and other underwater sites using inexpensive digital photographs and modeling software.