The Scottish Underwater Photogrammetry Unit (SUPU)
Stereophotogrammetry is a computer-based technique that generates highly complex three-dimensional point-cloud models using standard photo- or videographic source material obtained relatively simply and quickly. The models produce permanent records that can be analysed many times in multiple ways and can be measured in three-dimensions to much higher levels of accuracy than are possible using traditional methods.
We started using this technique underwater in 2013, while we were still the National Facility for Scientific Diving. Over the years we have greatly enhanced our delivery of underwater models through the employment of dedicated operators and the development of a networked cluster for photogrammetry processing. The photogrammetry cluster uses several dozen computing cores distributed across four workstation-class nodes to form a parallel processing supercomputer. This network architecture allows simultaneous or sequential alignment of tens of thousands of images and the creation of terabyte-scale projects.
Our delivery is now also dependent on imagery obtained through multiple capture techniques including diver-based stills, diver-based video, ROV stills and video, and aerial drone survey. We can now combine underwater models generated by diving or ROV, with above water ones constructed from aerial drone images to deliver models of complete coastal structures.
All of our models are ortho-rectified and scaled to permit accurate measurements to be taken from them.
The examples below are mostly animations made from the models. However, all the models can be fully manipulated in three-dimensions. Some models are presented on the SkethFab platform to demonstrate the versatility but in a lower resolution than is available in the full-scale models.
A – Pier 1 – Lismore: Combining models generated underwater by diving, with ones within the structure delivered either by snorkelling or small boat use, and with those made above water from aerial drone footage, produces a model of the complete coastal structure but where the water 'disappears'. View the Video »
B – Pier 2 – Underwater Craighouse: Even when diving-based surveys under piers are conducted in moderate or poor underwater visibility, the eventual models “remove” the seawater to provide a clear view of the total structure. View the Video »
C – Fine Scale Pier Pile: Where required, surveys can concentrate on isolated features and model them in very fine resolution. This model of a pier pile was constructed from images taken with a high-end digital-SLR camera and produced detail down to mm levels. View the Video »
D – Seabed Mapping: Making multiple passes over areas of seabed can produce 3-D views of broad-scale transects. Used for habitat mapping and impact assessment, the area of seabed that can be covered is constrained by depth (and, therefore, dive time) and visibility. View the Video »
E – Coral Reef 1 (broad-scale): The better water visibility usually associated with coral reefs means that relatively large areas of reef can be mapped quickly but accurately. Here, two 10 x 10 metre quadrats were completed in a single dive. Returning to the same sites over time produces indices of reef health and carbonate production. View the Video »
F – Coral Reef 2 (fine-scale / fluorescence): Scales and resolutions of surveys ae adapted to the main objective of the study. In this case, combining fine-scale habitat mapping with fluorescence photography, produces a method of quantifying and identifying coral recruits at much greater efficiency compared to traditional observational techniques. View the Video »
G – Artificial Reef: In temperate waters, it is often difficult to assess the complexity and structure of large natural or artificial reefs when water clarity is less than one metre. This is an example of an artificial reef that measures almost four metres high and over 10 metre across. The use of photogrammetry produced a model and allowed researchers to visualise an underwater target that was impossible to see in whole underwater. View the Video »
H – Chevron: Oil and gas platforms, whether in use or being prepared for decommissioning, attract biofouling. This is where plants and animals grow on the structures. Stereophotogrammetry supports the identification and volume (plus inferred mass) of the organisms that can inform management decisions. This is an example of fouling on a jacket leg of a structure in tropical waters and was generated from ROV footage of 'opportunity'. View the Video »
I – Bridge over the Atlantic: We also survey structures that are associated with, or close by, the coastal environment. This model was constructed totally from aerial drone footage of the iconic 'Bridge over the Atlantic' just south of Oban. The models produce permanent records of stuctures and comparisons over time periods can deliver records of consistent of reduced structural integrity. View the Video »