Abstract.

Monitoring the health of estuaries is very important to maintain the ecological balance of flora and fauna. Their survey has become fast and efficient thanks to technologies like light detection and ranging (LiDAR). LiDAR is used to construct digital terrain maps (digital surface models, digital terrain models), assess the state of coastal vegetation, and assess the ecological state of estuaries.

The sections will cover Introduction to LiDAR, The Importance of Estuaries, How LiDAR Can Help Estuary Research, and Applications of RESEPI Payload for Estuary Mapping. Finally, the benefits of using lidar in estuarine surveys will be briefly described.

Section 1. Introduction to LiDAR.

The first step is to understand the basic principles of LiDAR technology. LiDAR, which stands for light detection and ranging, works by emitting a concentrated light beam and measuring how long it takes for the light to return after hitting an object. When the LiDAR laser beam strikes an object, such as a tree or a building, part of the light is reflected to the sensor, as shown in Figure 1. By accurately timing the return of each laser pulse, the LiDAR system calculates the distance to each reflected point. This process uses the time-of-flight (ToF) method, which is based on the constant speed of light.

Figure 1. Conditional demonstration of LiDAR operation.

The collected points create a “point cloud,” a three-dimensional depiction of the scanned area of dots within a 3D space.

Differences among LiDAR models mainly involve variations in scanning angle, maximum range, and scanning precision. Many modern LiDAR devices provide several scanning modes. For example, the HESAI XT32 can modify rotation speed, scanning angle, and the number of returns, influencing the point cloud’s density and the laser’s capability to penetrate obstacles like trees and grass.

Integrating LiDAR with an inertial navigation system (INS) results in a system capable of georeferencing each point with centimeter-level precision and accuracy [1]. An example of such an integrated device is the Inertial Labs RESEPI Payload [2]. This compact device is fully customizable to accommodate various applications, offering options such as camera inclusion, LiDAR model selection, and GNSS receiver. These configurations are commonly utilized for drones and other small, unmanned aircraft systems (sUAS).

After collecting data with LiDAR payloads such as the RESEPI Payload, it is essential to process the raw data before creating the point cloud. Inertial Labs has developed specialized software called PCMasterPro [3]. This software simplifies post-processing, requiring minimal manual effort and ensuring smooth integration with hardware and software components.

Section 2. The Importance of Estuaries.

Before we discuss how lidar can impact estuary research and development, it’s essential to understand the current state of estuaries and their significance to ecology.

An estuary is a single-arm funnel-shaped river mouth extending towards the sea (Figure 2 [4]). This is where fresh and saltwater mix, and therefore, ecosystems mix. Estuaries provide ecosystems and refuges for many species of plants and animals, which depend on them to survive, feed, and reproduce.

Figure 2. Rio Estuary de la Plata (view from space ) [5].

One of its main characteristics is that it is one of the most productive ecosystems on the entire planet. Most organic matter is produced from the soil’s nutrients carried by rivers and oceans.

As a semi-closed system, it exchanges materials from several neighboring ecosystems. These are typically very shallow areas, meaning light can easily penetrate the water. Due to these environmental conditions, the estuary’s photosynthesis rate is relatively high. All this contributes to good primary production. It should also be remembered that estuaries are home to many species eaten by humans, such as crustaceans and mollusks, and some fish species, such as red snapper [6]. Marine animals often use estuaries as breeding grounds, where the young take advantage of the availability of abundant food during their first few weeks of life. Some plants growing in shallow estuaries can displace inorganic nitrogen compounds and metals from polluted land runoff waters, providing water filtration [7].

Estuaries can hold large amounts of water and prevent flooding, help prevent damage to coastlines during storms, and are essential in population management. In some cases, river flows carry more water, causing sediment and pollutants to be replaced. This more substantial flow keeps the water clean.

The economies of many coastal regions are centered around estuaries due to affluent populations of fish, shellfish, or algae. These are popular destinations for tourism; bird watching is very common in these areas, and they are places dedicated to scientific knowledge and education. However, a new study shows that almost half of the world’s estuaries have been modified by humans, and 20% of these estuary losses have occurred in the last 35 years [8]. The study results show that over the past 35 years, more than 100,000 hectares (250,000 acres) of estuaries have been converted to urban or agricultural land, with most losses (90%) occurring in rapidly developing Asian countries. Many high-income countries are now recognizing and addressing the damage, with places like the Tees Estuary in northern England investing in restoring mudflats and saltmarshes in the area to help reduce flood risk, build resilience to the climate crisis, replenish the fish population, and allow nature to recover [9]. Tees program Tidelands, which has received more than £30 million in funding, aims to rebuild flood defenses, restore mudflats and saltmarsh habitats, and remove tidal barriers so migratory fish can return to rivers where they have been absent for hundreds of years. The Tees Project Tidelands aims to create more than 50 hectares of mudflats, salt marshes, and other valuable estuarine habitats while reducing the risk of flooding homes and businesses now and in the future.

Examples of work performed include:

– Restoring tidal habitats such as salt marshes and mudflats, reducing flood risk and creating habitat for wildlife

– Reconstruction and improvement of existing flood protection structures

– Opening tributaries of the River Tees to tidal influence by removing barriers, allowing the river system and fish migration to function more naturally

– Restoration of Billingham Beck and Lustrum Beck in Stockton-on-Tees

Thus, regular monitoring is essential for environmental management and conservation of an important and rapidly changing site like the estuary.

Section 3. How Can LiDAR Help Estuary Research?

After familiarizing ourselves with estuaries’ importance to ecology, let’s examine how lidar can help survey them quickly and efficiently.

LiDAR technology allows you to quickly scan large areas and obtain a digital map of the region, with which you can quickly assess changes in the landscape. This, in turn, will help to immediately take measures to strengthen the coastal zone or conduct watershed conservation operations. LiDAR will also provide the most comprehensive data possible and create a high-resolution baseline of topographic conditions in the project area so researchers can monitor sedimentation and erosion that may affect the estuary’s overall health.

The list below describes the key benefits of using lidar to scan estuaries.

• When restoring salt marshes, they often resort to reclamation [10]. A digital map of the area will help design a drainage system to maintain groundwater at the required level to prevent secondary salinization.
• It is very important for researchers and scientists to estimate the volume of deposits resulting from the movement of river waters to the sea. The sedimentation of large (silty and sandy) suspended grains occurs due to a sharp decrease in the velocity of the carrier flow, which reduces the vertical component of turbulence and sedimentation of particles. There is third-party software that will allow you to quickly measure the volume of an area of interest, such as LiDAR 360 [11, 12].
• Thanks to dual returns, LiDAR can scan the area under the tree canopy, allowing the user to obtain information about the terrain of coastal forests and vegetation and effectively manage the planting of new vegetation or cutting down unnecessary vegetation, which is one strategy for protecting against flooding and coastal erosion [13, 14].
• LiDAR data will help assess the condition of existing protective structures and carry out repair work on time.
• Thanks to the camera equipped with RESEPI, the user can also obtain photographs of the area to assess the ecological state and photogrammetry, if necessary, visually. This will help evaluate the level of environmental disturbance. It can be determined by the level of changes in the parameters of the ecological state of the elements of the coastal recreational zone: changes in the width of the beach, the rate of coastal abrasion under conditions of recreational use, the risk of landslides, the level of pollution of the coastal zone of the sea, the level of contamination of soils and soils of the territories of the recreational zone, the level of atmospheric air pollution, anthropogenic loads (building density), technical condition of bank protection structures, and wastewater disposal systems.
• It does not require direct site presence, which saves a considerable amount of time and money.

Section 4. Application of RESEPI Payload for Estuary Mapping.

Back Forty Aerial Solutions, in conjunction with the Baruch Marine Field Research Laboratory at the University of South Carolina and the Department of Geography at the University of South Carolina, and grants from the National Oceanic and Atmospheric Administration (NOAA), conducted a mission to map nearly 400 acres of the Crab Hole Estuary and Goat Island [15].

The primary objective was to obtain an accurate digital elevation model (DEM) of tidal Crab Hole Creek at low tide. Researchers can use this model to calculate the amount of nutrients and sediment deposited by tides through the creek beds. To get complete data, a time was chosen when the tidal creek basin was nearly empty – February 22, 2023. This allowed the researchers to obtain complete topographic data with high accuracy.

Figure 3. RESEPI payload to fly over Goat Island.

A secondary task was to test two different payloads, RESEPI XT-32 and RESEPI Avia [17, 18]. This task was equally important as the two lidars differ in viewing angle, number of returns, and scanning patterns. Using data from the two different lidars, it’s possible to assess the scanning quality of vegetation, particularly spartina. RESEPI Avia was chosen for its high point density, triple returns, and multiple scan patterns. RESEPI XT-32 was selected for its wider field of view, 32 lasers, and lower noise profile than Livox Avia [19]. Due to the characteristics of each lidar, particularly scanning patterns and different viewing angles, researchers collected data from different point cloud densities and angles. Line scan patterns were used to create accurate topographic elevation models (DEMs), and a spirograph pattern was used to collect detailed vegetation data to create vegetation biomass models.

After installing ground control points (GCP), Figure 4, and control images, based on which the received data was refined, the Emlid Reach RS2 base station was configured [16].

Figure 4. Ground control point.

Conventionally, the mission was divided into 2 phases:

– Phase 1 had two objectives: collecting data along Goat Island using two RESEPIs Payloads with different configurations to determine the best one for phase 2 and the Spartina research. Aboard the DJI Matrice 300, the team completed several flights using UGCS flight control software. The drone performed repeated missions over Goat Island and adjacent swamp test sites at 55 m above ground level, at a speed of 5.5 m/s, with a lateral overlap of 30%, respectively, of each payload’s ideal field of view.

– Phase 2, based on Phase 1 data, was conducted at up to 60 m above ground level to compensate for flight time while increasing lateral overlap to 45% for the RESEXT-32 to improve vegetation penetration in dense clumps of spartina.

Results

Once the flights were completed, the data was processed in PCMasterPro to produce point clouds and trajectories. Third-party software was used for soil classification, quality control, alignment, and DEM and TIN generation. Figure 5 shows the raw Crab point cloud Haul Creek obtained with RESEPI XT-32.

Figure 5. Crab point cloud Haul Creek obtained with RESEPI XT-32.

The project was completed, and thanks to this, the researchers obtained an accurate DEM model of the estuary, Figure 6. The high-quality lidars used in RESEPI Payload produced high-density point clouds with excellent vegetation penetration. It was also discovered that the Livox Avia showed better spartina scanning results than the XT-32. Comparatively, the XT-32 is better at scanning the height and sides of vegetation because its field of view is more expansive and has low noise. Thus, combining two different lidars made it possible to obtain a high-quality digital elevation model and scan dense vegetation to fully represent the estuary and improve the model’s accuracy and calculations.

Figure 6. Digital terrain models.

LiDAR technology provided exceptional data density and accuracy, allowing for accurate modeling of river channels and volumetric calculations of nutrient deposition. This data is essential for monitoring estuary health and temporary ecosystem changes associated with climate change and sea level rise.

Before using RESEPI Payload, the user must have a drone and flight license, a base station, and a PCMasterPro software license for data processing [16].

Conclusion.

Estuaries are a vital ecosystem rich in nutrients, flora, and fauna humans use for food and production. They are protective barriers against flooding, shoreline damage, and natural filters that purify water.

Due to human activity, many of these natural sites have been lost due to the use of coastal regions for constructing cities and industries. In this regard, many countries are rolling out programs to restore estuaries, investing large sums of money.

Therefore, organizing regular monitoring and research work in existing estuaries is very important to effectively manage environmental change and conserve such essential and rapidly changing places. This is where LiDAR data collection and processing can help. Unlike manual methods and photogrammetry, LiDAR data collection is much faster, and the user receives a 3D point cloud rather than 2D, uninformative photographic images. Thanks to this, you can solve a wide range of problems: monitor the state of soil changes thanks to digital terrain maps (digital surface models, digital elevation models), design drainage systems to control salt marshes, and calculate the volume of deposits and nutrients. This information will help plan work for scientists and researchers studying estuaries to maintain their ecology.

As we demonstrated with the estuary exploration mission, payloads like the RESEPI Payload make monitoring fast and efficient. With RESEPI Payload, you can quickly obtain a digital map of the area and photos for photogrammetry without resorting to manual methods and expensive equipment.

Inertial Labs is committed to providing high-quality solutions with customization and excellent value for money at an affordable price.

References.

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[13] Mendez, Maria. “Using LiDAR in Forestry Management.” RESEPI, 24 June 2024, lidarpayload.com/using-lidar-in-forestry-management/. Accessed 23 July 2024.

[14] Wikipedia Contributors. “Riparian Zone.” Wikipedia, Wikimedia Foundation, 25 Sept. 2019, en.wikipedia.org/wiki/Riparian_zone.

[15] Solutions, Back Forty Aerial. “Case Study: Drone LiDAR Mapping 400-Acres of a National Estuarine Research Reserve.” Back Forty Drones, 18 July 2024, www.backfortydrones.com/post/case-study-drone-lidar-mapping-400-acres-of-a-national-estuarine-research-reserve. Accessed 23 July 2024.

[16] “Buy Reach RS2+ | Buy Multi-Band RTK GNSS Receiver.” Emlid Store US, store.emlid.com/products/reach-rs2-plus?variant=47253689467185. Accessed 3 June 2024.

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