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Faafu Atoll: above and below
An insight of three Maldivian islands
Travel routes from all participants to Malé
Introduction
BridgET is an ERAMUS+ funded project, an initiative created by the European Union to promote international mobility and collaboration in the field of higher education. Alessandra Savini from the University of Milano-Bicocca describes the aim of the project as to “bridge the gap between land and sea through a virtual platform that enables innovative teaching and community engagement in the science of climate change-induced marine and coastal geohazards.” The project brings together leading institutions from across Europe, including the Kiel University, University of Malta, University of Milano-Bicocca, University of Liége, Arctic University of Norway, and University of Athens. Additional collaborators include Orthodrone GmbH, INGV and INAF. BridgET organizes a series of summer schools aimed at Master students specializing in geosciences. These programs offer hands-on learning experiences in geologically significant locations. In 2022, the summer school was held on the volcanic island of Santorini, Greece. In 2023, students explored Mount Etna in Sicily, Italy. This year (2024), the program took place in the Maldives, where participants studied geohazards in a tropical marine environment.
Speed boat travel from Malé to Magoodhoo island
The Maldives is an archipelago made up of a double chain of coral atolls scattered across the central region of the Indian Ocean. The Republic of the Maldives includes approximately 1200 islands. The participants arrived at Malé International Airport, located in the eastern chain of atolls, and traveled across the Inner Sea on a three-hour speedboat ride. They reached the Faafu Atoll in the western chain and arrived at Magoodhoo Island, where the MaRHE Center is located. Magoodhoo is an inhabited island with a population of about 800 people. Islands in the Maldives are geographical organized in inhabited islands, resort islands, or uninhabited islands.
Impressions taken from the three different islands
During the summer school, students had the opportunity to visit examples of each island type, providing insights into the region’s diverse characteristics. The organizers provided an introduction to the project, followed by a tour of Magoodhoo Island. Throughout the program, various lessons were conducted, focusing on techniques for mapping the land-to-sea interface and explaining the different methods used in this process. Students had a real on-field experience and the chance to use the different methods and technologies such as: -MBES and ROV from the Dhoni boat; -aerial photogrammetry with drones; -underwater photogrammetry; -experience Virtual Reality. For each method, it was explained how to process data and actually create maps with different resolutions and how to merge them to fill the gap between land and sea. Scroll down and enjoy the narration
Drone Survey
As part of the Bridget Summer School in Maghoodoo, Maldives, drone surveys play a big part in bridging the gap between land and sea, embodying the project´s mission. By utilizing new technologies, the students explore coral reefs and coastal morphology with unprecedented clarity and efficiency.
Mapping the interface: Land to sea
Using drones equipped with RGB multispectral cameras and lidar, students conducted aerial surveys over the coastal line of Magoodhoo and the surrounding coral reefs. The goal was to map the intricate features of these environments in high resolution, providing a detailed foundation for understanding their interplay and vulnerabilities.
The colour of the points indicate the number of surveys conducted.
How to plan and conduct “drone surveys”
• Plan flight paths and optimize coverage using the drone (DJI) built-in software. • Operate drones to capture overlapping images necessary for creating seamless photogrammetric models. • Always keep eye contact with the drone, to keep the flight safe!
Left: preparing the drone, middle: the survey drone, right: the screen of the controller and the survey mission.
The video shows a takeoff of a survey drone.
Post-processing
The collected data is loaded into the Metashape Pro software which; • Aligns overlapping images captured by drones, using a process called “Structure-from- motion(SfM)” • It automatically filters out poor-quality photos • It identifies shared features between images to create a seamless, georeferenced mosaic of the surveyed area. • Creates a result as orthomosaics and DEMs. Orthomosaics are high-resolution, geometrically accurate maps of the area. DEMs are “digital elevation models”, which are critical for assessing geohazards.
3D model of a drone survey area.
Orthomosaic map of the entire surveyed area.
In the map below, the different survey areas can be seen combined. As you can see, the resolution is much higher than of a typical satellite image.
The method can also be used to map patches of corals, instead of getting in the water yourself. In the next section, you can read more about this.
3D model of a coral reef
Underwater Photogrammetry
After seeing the island from above you will want to have a closer look… When you are in a place like the Maldives you will want to see the underwater life and colorful corals while snorkeling. This is why the next point is underwater photogrammetry.
Here are some tips to conduct underwater photogrammetry;
- Specialized equipment such as a high-quality underwater camera is used
- It is important to use a standard setting, avoiding the fisheye effect
- The study area should be clearly defined, often with the help of drones, while taking into consideration environmental factors (i.e. currents)
- Visible, flat markers placed on the coral are crucial for photo alignment during processing and for providing scale references
How to take the perfect pictures? | Why is this important? |
---|---|
Maintain consistent positioning | Same scale and distance between the object and the camera needed |
70% overlap between photos | To help the software align |
At first, the angle of the photos must be horizontal | To have a plane view of the area |
Then different angles: overhead, side, upward | to capture texture for the 3D model. |
Underwater photogrammetry footage from the field
As with the drone survey Metashape is utilized in photogrammetry.
However, survey mistakes may still cause alignment issues, so attention to detail is necessary. Once the photos are aligned, targets and markers are used to establish scale, and a point cloud is generated by identifying similar points between images.
From the point cloud, a 3D model is created, along with an orthomosaic and a digital elevation model (DEM). Tiled models can also be produced for VR applications. Furthermore, the produced DEM provides relative depths, not absolute altitude, as GPS data is not typically available underwater. The goal is to capture the optimal number of images to create the largest point cloud possible while minimizing processing time.
This technique is very useful to visualize the underwater world but only in shallow water. If you want to explore deeper, you will need to use different techniques like Multibeam and ROV.
Underwater photogrammetry survey preparation (placing markers around the coral)
To see the 3D models of corals processed, click the locations in the map below.
Locations of the underwater photogrammetry survey.
Let's see a part of the reef and some corals like a 3D model !
BRIDGET_3D_Patch_reef
Unveiling Hidden Landscapes: Multibeam Echosounders and Remotely Operated Vehicles
MBES survey area.
Multibeam (MBES) is a powerful method used to map the floor of deep and shallow water bodies with high resolution. It employs a transducer that emits an acoustic pulse which is reflected by the seabed and is picked up by an array of receivers, positioned at varying angles. With knowledge of the sound velocity in the water column, the two-way travel time of the sound pulse can be used to calculate the depth.
Bathymetry of the surveyed area of Magoodhoo lagoon.
Depending on the swath angle of the acoustic beams, a bigger footprint of the seabed can be measured. It usually falls within the range of 120° to 170°, resulting in footprints that can range from 3.5 to 25 times the water depth. Several sound frequencies are used for a variety of depth ranges. High frequencies are absorbed faster in the water column but lead to higher resolutions; therefore, high frequencies are used in shallow waters only, while low frequencies must be used for the deep sea.
The data collected can be processed in software such as Qimera. Using these applications, it is possible to correct errors from the various movements of the vessel, the tides and other factors. It is also possible to remove outliers in the data in this kind of software. In completing the post processing, the data becomes ready for analysis. The results of bathymetric data can be studied to identify marine geohazards, past geological events, marine habitats, archaeological sites and other features. MBES reaches its limits closer to the shoreline, where vessels can not navigate due to shallow depth. Therefore, other methods are needed to close the gap between land and sea. Sometimes, small-scale bathymetric instruments are remotely piloted on tiny vessels or by using drones to map shallow regions.
3D model of bathymetry
The data was acquired on "the Dhoni", a small vessel with a pole-mounted MBES. It can reach areas, that are even less than 5 meters shallow.
Remotely operated vehicles (ROV) are robots that can reach areas that are otherwise inaccessible to humans. They have a control unit, operated by a person on the coast or on board a vessel. The ROVs are usually equipped with high-resolution cameras that send live imagery to the operating professionals and are fitted with lights and additional attachments, such as robotic arms to collect samples of sediments, water, gas or biogenic substances. As a result, they range in size according to their application. They are attached to the operating console by a cable which communicates the movements and relays data back. The ROV pilot can direct the ROV to move forward, backward, up, down and rotate. The camera can be controlled by the operator as well; it can be moved from forward facing to nadir. These functions allow the pilot to safely maneuver the ROV. Especially in deep waters, combining MBES and ROV data can provide a holistic understanding of the seafloor structure and environment.
Acquiring the data with a ROV
The ROV we used is a small and handy device. It can be launched from a vessel or the shore and is easy to set out and retrieve:
ROV in action! (Credits: Adrian Andreassen)
Explore the different locations in the following web application. Use the arrows at the upper left or select a location on the map to navigate around.
Attachment Viewer
Underwater impressions
Don't forget that our actions above the sea surface affect the life below it!