What is Photogrammetry?

Photogrammetry refers to the scientific approach of physical data collection of objects and their environment. This process is done through the interpretation of photographs based on image overlap and parallax calculations. Stereoscopic parallax refers to the apparent displacement in the position of an object, with respect to a frame of reference, caused by a shift in the position of observation. The differences in the parallax of various objects of interest (called differential parallax) can be used to measure the heights of objects and to extract topographic information (source).

What is Photogrammetry Used for?

Mapping

Specific software can use triangulation (the tracing and measurement of triangles on an image to determine relative positions and distances of points) to create detailed maps from aerial photos. Once common figures are identified between two or more aerial images, the process can begin. As the images continue, piece by piece, an overhead map can be produced depicting a larger surface area, so long as there is commonality to use as reference between sequential images. Photogrammetry in mapping has become increasingly common as aerial drone technology advances. Modern aerial images are reliable, highly accurate, and produce rapid results without traditional ground surveying.

Archaeology

Being able to use imagery to create 3D models of complex structures is extremely beneficial in archaeology. Modeling historical buildings, damaged or collapsed structures, catacombs, artifacts, etc., provides archaeologists detailed insights into these environments. These physical recordings act as a method of keeping records, either for historical purposes or to monitor degradation over time. Additionally, creating these digital models also allows for knowledge sharing around the globe, providing an economical solution for smaller archaeology teams to view the findings remotely. DESCRIPTION HERE

How ROVs Improve Photogrammetric Modeling?

Similar to how aerial drones assist in mapping locations on land, ROVs are an effective tool at capturing reliable images underwater. Compared to dive teams or unmanned vehicle systems, ROVs have superb navigational flexibility. With some units operating off battery power, workers can deploy the ROV from virtually anywhere, and advancements in design and station holding capabilities allow for consistently clear imaging. This ‘plug-and-play’ robotic solution allows for work to begin nearly instantaneously, and empower the topside team to have an ongoing visual of what the ROV is seeing.

Pivot Exploration

Additional benefits include the ability to integrate with high frequency imaging sonars. In low visibility conditions, sonar footage can be overlayed with images, or in some cases, used alone to create a photogrammetric model.

Stantec Using Deep Trekker ROVs for 3D Archaeological Modeling

The Main Challenges of Underwater Surveying

Water Visibility
Unlike air, which is generally clear for capturing photographs, water is very subject to turbidity. Dirt, sediment, marine growth, etc., can all compile for low visibility environments. This is particularly true for shallow freshwater environments.
Water Hazards
On top of visibility issues, water can also come with substantial physical hazards. Frigid temperatures, high current, or dangerous objects can make it extremely difficult or dangerous for humans to enter the water for surveying.
Third Party Teams
Traditionally, the most common method of photographing underwater structures is contracting dive teams. These divers enter the water, take as many images as possible, and resurface to communicate their findings with archaeologists or topographers. With detailed photogrammetric models requiring hundreds or thousands of photographs, any breakdown in communication can quickly snowball into large-scale slowdowns. Additionally, divers are limited by their oxygen, as well as pressurization during bounce dives, which can extend the mission severely.

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Mission Objective

Stantec Markham’s archaeological department was recently tasked with evaluating a dam structure in Nassau Mills in Peterborough, Ontario. This site housed an active dam that was approaching the end of its functional lifespan, as well as the historical remains of a previous dam from the 1800s. The goal of this survey was to examine the historical remains, while also identifying the construction methods used in past dams for new building plans.

The Equipment Used

Deep Trekker’s REVOLUTION and PIVOT ROV were used in conjunction to provide extended battery life to last through the whole workday. Both of these devices are equipped with six thrusters, as well as rotating camera heads and tool platforms. Four of the six thrusters are vectored, while the remaining two are vertical, allowing for lateral and vertical movements without adjusting the ROV pitch. The only other piece of equipment used was a GoPro camera, which was used in conjunction with the vehicle cameras for additional imaging. This camera was simply grasped between the grabber claw of the ROV when in use.

Pivot Operator

Phase 1: Imaging

The first phase of Stantec’s photogrammetric modeling was the gathering of high-resolution images. The first challenge this project faced was weather. The survey needed to be conducted during Canadian winter, with average low temperatures of -11 degrees Celsius for the area. Sending divers in at these temperatures would require extensive gear, and result in shorter dive times to limit exposure risk. Secondly, working near a dam structure produces high current and risks of differential pressure. Project Archaeologist Mike Maloney stated, “if we had divers down, we're very close to a dam there'd be a lot of safety concerns getting the work done”. To address these issues, Mike alongside Darren Kipping, another archaeologist at Stantec, decided an ROV was the best tactic for imaging. By equipping an ROV with a GoPro camera set to take JPEG images every two seconds, the duo was able to pilot the ROV in a grid pattern for extensive photo coverage from varying angles. This tactic was able to capture near continuous images by field swapping between two vehicles.

Controller The pair stated that they were able to conduct the underwater survey much quicker than usual. Noting that “normally if we had divers down, we as the archaeologists would instruct them what to be looking for what to record and then they would come back up and kind of inform us what they've seen”. In a single afternoon, the team was able to capture all the imagery needed to move onto the second stage; modeling.

Phase 2: Modeling

Photogrammetry modeling

Once the necessary imagery was captured, Mike and Darren were able to upload their findings, alongside some previous side-scan sonar footage to generate photogrammetric models. The software used was ‘Agisoft Metashape’. Agisoft Metashape is a stand-alone software product that performs photogrammetric processing of digital images and generates 3D spatial data to be used in GIS applications, cultural heritage documentation, and visual effects production as well as for indirect measurements of objects of various scales.

Photogrammetry Overview

The figure above demonstrates the capabilities of precise measurements found in the modeling capabilities. The images captured were able to render completely interactive models down to millimetric accuracy. Once finalized, Stantec was able to compare the photogrammetric renderings against historical documents to track the impact of environmental factors over time.

The Deep Trekker Difference

Rotating Camera Heads

Deep Trekker’s rotating camera heads enable operators to rotate between 220 and 270 degrees. This allows for extremely smooth and consistent imaging since the pitch of the ROV requires no adjustment for downward facing photographs. During Stantec’s survey, they were able to easily capture ‘aerial’ positioned photos, as well as adjust the rotation for complex sections as needed.

Portability

Accessing the deployment site required the team to navigate across a system of narrow walkways. Thanks to Deep Trekker’s battery operated ROVs and handheld controller, there was no need to travel with external generators or control consoles. This drastically increases the maneuverability during remote surveys, as well as allows for convenient air travel as checked luggage.

luggage

Station Holding

Utilizing Deep Trekker’s in-house BRIDGE software, the ROVs are incredibly intelligent at station holding through currents. With auto-depth and auto-heading capabilities, piloting and capturing consistent images is extremely intuitive. At Nassau Mills Dam, the current was substantial, however the PIVOT and REVOLUTION ROVs were able to smoothly navigate through the required grid pattern.

Key Takeaways

Using Deep Trekker ROVs, Stantec was able to quickly, safely, and easily capture all the photos needed for modeling in a single day. The speed in which two team members were able to complete a task like this was remarkable. By deploying ROVs, the pair of archaeologists were able to personally direct the vehicles to exactly where they needed to be, rather than communicate back and forth with a dive team. By empowering the end-user to capture the required images, they were able to successfully create high-detail 3D photogrammetric models of Nassau Mills dam, with no risk or repeating costs. After the project concluded, Stantec Markham immediately committed to the purchase of a PIVOT ROV for future modeling projects.