Case Study: Drone LiDAR Mapping 400-Acres of a National Estuarine Research Reserve

Project Overview

Back Forty Aerial Solutions, a drone mapping firm based in Blythewood, South Carolina in partnership with the University of South Carolina’s Baruch Marine Field Laboratory, University of South Carolina Dept of Geography, and grants from the National Oceanic and Atmospheric Administration (NOAA), undertook a two-phase drone LiDAR mapping project, to map nearly 400-acres of estuary in a tidal creek basin know as Crab Haul and adjacent area known as Goat Island. Crab Haul and Goat Island are just a ‘small’ section of the North Inlet–Winyah Bay National Estuarine Research Reserve (NIWB NERR) which is located just East across the Winyah Bay from Georgetown, South Carolina and is largely encompassed within the expansive Hobcaw Barony. 

Map of the NERR, the combined areas of Winyah Bay and the North Inlet

Together the Hobcaw Barony and NERR preserve 18,916 acres of tidal and transitional marshes, oyster reefs, beaches, coastal forest, and open water. “Through research, education, stewardship and training, the Reserve promotes healthy estuaries, watershed preservation, resilient coastal communities, and thriving ecosystems.” The primary goal of our main project was to create an accurate digital elevation model (DEM) of the Crab Haul tidal creek basin at low-king tides for researchers to use in calculating the volume of potential nutrient and sediment deposits flowing into it through the stream channels, and high tidal events. Led by the Laboratory's director, Dr. Erik Smith, BFAS aimed to use this small window of time on February 22nd, 2023 during this annual perigee tidal event that would nearly empty the tidal creek basin as to capture the most complete data and create a high resolution baseline of topographic conditions in the project area for researchers to track sediment deposition and erosion that could impact the estuary's overall health. 

Additionally, we sought to test different lidar scanners and scan patterns, to determine if any were able to pick up the fine spartina grasses growing in the marsh, which Dr. Susan Wang of USC Dept of Geography, is tracking in her research of their potential decline in various parts of the NERR as sea levels rise. We at Back Forty absolutely embrace working with scientific researchers, because we share a common ethos of leveraging the scientific process and cutting-edge technology to understand the environment around us. If you are a researcher reading this and have a project that you’re interested in leveraging our expertise, equipment, or post-processing methodologies on, we want to hear from you! We provide deep discounts on services to the academic community because we understand your funding constraints and want to partner with you in pursuit of your scientific research, in lieu of profits. Please, don’t hesitate to reach out to us at contact@backforytdrones.com. Keep reading to learn more juicy details about this project!

BFAS drone, equipped with a LiDAR/Camera payload mapping Goat Island

Methodology

Base station set up on survey monument and flight operation from Oyster Landing

The first part of any lidar mapping project BFAS conducts is finding a locale for establishing tight geodetic control, preferring minimal baseline distance between the base station GNSS receiver that will collect static, 1 second observations over a known point, and the project area where ground control points (GCPs) and checkshots are located as a methodology for further vertical and horizontal refinement of the resulting datasets. These known locations will then be later used to process a trajectory that the lidar payload travels, and reference the data within a goal of <=5cm of vertical error as measured against the GCPs and checkshots. We were in luck- the NERR has several Class V1H1, NGS Monuments on site. For Phase 1, we were working on Goat Island, and selected the closest V1H1 monument, PID DD1355, on Clambank. For Phase 2, we selected the “Oyster Set” monument at the North end of Crab Haul Creek on Oyster Landing. However this monument was obscured by several large clumps of vegetation and would block part of the sky from the receiver, potentially degrading the quality of our trajectory processing.

We then chose a nearby survey monument established by the NERR on Oyster Landing, that has not yet been added to the NGS register, to set up our Emlid Reach RS2 GPS receiver for static observation during our low-king tide flights. GCPs and checkshots were provided by Dr. Smith’s team using a Trimble R8, in a base-rover configuration from the same Oyster Landing survey monument to create a common origination point for the Crab Haul data. Luckily for us, Dr. Smith has established a truly unique, and extensive network of permanent GCPs that stand up to the harsh coastal conditions, and are elevated above the tidal zone. 

Elevated, permanent GCP threaded on a metal rod driven 6 feet into the marsh

Previous researchers at the NIWB NERR have constructed simple boardwalks in the marsh, which checkshots were also collected on by Dr. Smith’s team to further assist in the fine alignment of the point cloud across such an expansive area. Without these existing infrastructure elements on site, this project would have proven to be an extremely difficult project to establish reliable control on due to the soft and ever changing nature of the pluff mud and constant tidal forces.

Having control accounted for, we then set out to determine what lidar payload in our inventory would provide us with the best possible point density, with low noise, in the moist pluff mud and dense spartina marsh grass. 

BFAS owner, Eric Harkins, somehow not falling off one of the boardwalks used by researchers

Equipment

Working with Dr. Susan Wang, of USC’s Dept of Geography, we began Phase 1 in September, 2022. Phase 1 had two goals: performing data collection along a spit of land known as Goat Island to collect data for Dr. Wang’s spartina research, and get sample datasets of our two lidar payloads in different configurations to determine best fit for Phase 2 data collection. BFAS owns two Inertial Labs lidar payloads. A RESEPI Livox Avia, and a RESEPI Hesai XT32, both equipped with co-aligned 26MP Mapping Camera, 16mm Lens, Single Antenna NovAtel IMU + GNSS Receiver, and Inertial Lab’s high-precision INS. Flown on a DJI Matrice 300, the team performed multiple flights using UGCS flight control software. The drone flew repeat missions over Goat Island and adjacent marsh test plots at 55 meters AGL, at 5.5 m/s, with a sidelap of 30%, respectively of each payload’s ideal field of view. Findings from Phase 1 precipitated changes in the mission planning for Phase 2, with an increase in altitude to 60m AGL, to offset flight times of the increase to 45% sidelap for the Hesai XT32 to improve vegetation penetration in dense spartina clusters.

David Henderson, of BFAS, checking SCRTN corrections accuracy on the DD1355 monument

The RESEPI Livox Avia, a lightweight, single-laser LiDAR scanner, was chosen for its high point density, triple-returns, and multiple scan patterns. The RESEPI Hesai Pandar XT 32, a high-performance LiDAR scanner with dual-returns, was selected for its wider field of view, 32 lasers and lower noise profile versus the Livox Avia. Using these scanners and their respective scan patterns allowed us to collect data from various angles and point densities. The linear scan patterns are most commonly used for creating accurate topographic terrain models (DEMs), while the spirograph pattern was hypothesized to capture detailed vegetation data, essential for modeling ground vegetation biomass. Both sensor datasheets can be viewed in detail in the hyperlinks above.

Data Processing

Upon completing the data collection, the collected data was processed using BFAS’s proprietary workflows that employ several softwares. PC Master for the trajectory, Rock Cloud for initial data visualization/inspection, preliminary GCP alignment, and coordinate system transformation. TerraScan for ground classification, LP360 for manual QC and final alignment, and lastly Global Mapper for DEM and TIN generation. 

Crab Haul Creek - Ground classified points visualized by height in Global Mapper

Crab Haul Creek - TIN Surface generated from 1 foot grid DTM in Global Mapper

Successes

The drone LiDAR technology provided exceptional data density and accuracy, enabling precise modeling of stream channels and volumetric calculations of nutrient deposits. This data is essential for monitoring the estuary's health and temporal changes to the ecosystem linked to climate change and rising sea levels.

The project successfully met its objectives, resulting in an accurate DEM model of the estuary, exceeding Dr. Smith’s expectations for point density and vegetation penetration. For Dr. Wang’s research, we also determined the Livox Avia in the spirograph configuration resulted in more returns in the spartina at her control plots near Goat Island, although it struggled with new growth. The Hesai XT32 also had higher returns of spartina height, but at the wider angles of its field of view, due to angle of incidence at the more oblique angle. It did a poor job of picking up finer vegetation in the more nadir angle range, but had excellent vegetation penetration and low noise, as much as can be expected in a landscape pockmarked by oyster beds and fiddler crab holes. Overall, the combination of different LiDAR scanners and scan patterns provided a more comprehensive view of the estuary, improving the accuracy of the 3D model and volumetric calculations.

Crab Haul Creek - Raw point cloud showing GCPs & checkshots, visualized in Rock Cloud with a 2 meter elevation band highlighting the vegetation and fine elevation changes captured by the RESEPI Hesai XT32

Conclusion

The successful use of drone LiDAR technology in mapping this nearly 400-acre area for the Baruch Marine Field Laboratory and NOAA highlights the technology's potential in environmental monitoring on a landscape level. It also highlights the importance of durable, elevated, control points in constantly changing hard to access environments such as the nation’s 30 NERRs. Back Forty Aerial Solutions provided highly accurate and dense topographic data, allowing for a confident baseline to conduct future research of the tidal ecosystem and climate change studies in the bay that drains the third largest watershed on the US East Coast. This case study underscores the value of testing multiple LiDAR scanners and scan patterns to enhance data collection methodologies for environmental monitoring and management strategies. It also highlights Back Forty’s willingness to bring all of its experience and assets to bear in pursuit of tangible results and paradigm shifts in the application of LiDAR and UAV technology for the grant-dependent scientific community.

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