Underwater Videography
Underwater Videography: An Innovative Way to Assess Oyster Reefs
We use custom-made underwater videography systems to map the seafloor for a variety of purposes. In addition to mapping oyster reefs, recent projects include assessments of seafloor conditions relevant to macroalgal production in estuarine and shallow shelf waters in southwest Florida, coral habitats in the United Arab Emirates, and shallow shelf communities in the Gulf of Maine (see reports and presentations). For oyster reefs, we use both towed and “drop” systems to determine the boundaries and general characteristics of the reefs. The cameras are made by Seaviewer underwater video systems, and include several different models. One system has dual LED lights, and is capable of working in water depths up to about 50 m. The sleds include two models differing in the size and weight, and the cameras can be installed on any of the deployment systems. The onboard components include a digital video recorder, GPS overlay unit that imprints position data onto the imagery, color monitor, and 12-volt battery that powers everything; all enclosed in a water-tight case.
In some cases, underwater video can provide a low-cost alternative to single beam sonars (and other methods) for seafloor mapping. In fact, towed video has become the method of choice for periodic mapping of boundaries and general condition of the oyster reefs in New Hampshire for management purposes (see Grizzle et al. 2005 and 2008 for studies comparing various sonars to video and other methods for mapping oyster reefs). However, there are limitations to its use. The most obvious is that water clarity must be sufficient to allow unambiguous identification of the features being mapped. We have found that even in estuaries where turbidity levels are typically quite high, there are times when underwater visibility is sufficient for obtaining useful video imagery. Another limitation is tow speed. Useful imagery is typically limited to tow speeds of about 2 knots, a bit faster when the water is clear but less under turbid conditions. In any case, this limits the linear distances that can be surveyed on an hourly basis, and thus affects the cost of each project.
Another potential issue with video mapping is positional accuracy. We typically use GPS units that have advertised positional accuracies of <3 m. We could use instruments with greater accuracy but the GPS systems are not the primary control on positional accuracy; it is the position of the camera relative to the GPS receiver. In general, the shallower the water the easier it is to maintain the position of the camera directly under the GPS receiver which is onboard the vessel. As water depth increases the ability to control the position of the camera relative to the GPS receiver decreases. Thus, we have found that positional accuracies of 5 to 10 m are more realistic for seafloor maps mainly based on towed underwater video.
If you think underwater videography could be used to meet any of your mapping or other underwater imaging needs please contact us.
We use custom-made underwater videography systems to map the seafloor for a variety of purposes. In addition to mapping oyster reefs, recent projects include assessments of seafloor conditions relevant to macroalgal production in estuarine and shallow shelf waters in southwest Florida, coral habitats in the United Arab Emirates, and shallow shelf communities in the Gulf of Maine (see reports and presentations). For oyster reefs, we use both towed and “drop” systems to determine the boundaries and general characteristics of the reefs. The cameras are made by Seaviewer underwater video systems, and include several different models. One system has dual LED lights, and is capable of working in water depths up to about 50 m. The sleds include two models differing in the size and weight, and the cameras can be installed on any of the deployment systems. The onboard components include a digital video recorder, GPS overlay unit that imprints position data onto the imagery, color monitor, and 12-volt battery that powers everything; all enclosed in a water-tight case.
In some cases, underwater video can provide a low-cost alternative to single beam sonars (and other methods) for seafloor mapping. In fact, towed video has become the method of choice for periodic mapping of boundaries and general condition of the oyster reefs in New Hampshire for management purposes (see Grizzle et al. 2005 and 2008 for studies comparing various sonars to video and other methods for mapping oyster reefs). However, there are limitations to its use. The most obvious is that water clarity must be sufficient to allow unambiguous identification of the features being mapped. We have found that even in estuaries where turbidity levels are typically quite high, there are times when underwater visibility is sufficient for obtaining useful video imagery. Another limitation is tow speed. Useful imagery is typically limited to tow speeds of about 2 knots, a bit faster when the water is clear but less under turbid conditions. In any case, this limits the linear distances that can be surveyed on an hourly basis, and thus affects the cost of each project.
Another potential issue with video mapping is positional accuracy. We typically use GPS units that have advertised positional accuracies of <3 m. We could use instruments with greater accuracy but the GPS systems are not the primary control on positional accuracy; it is the position of the camera relative to the GPS receiver. In general, the shallower the water the easier it is to maintain the position of the camera directly under the GPS receiver which is onboard the vessel. As water depth increases the ability to control the position of the camera relative to the GPS receiver decreases. Thus, we have found that positional accuracies of 5 to 10 m are more realistic for seafloor maps mainly based on towed underwater video.
If you think underwater videography could be used to meet any of your mapping or other underwater imaging needs please contact us.
