eNews

#04 2024

Investigating and surveying underwater forests: Studying the kelp dispersal areas, density and biomass along the South African West Coast

This case study was first published on the GeoLas website (https://www.geolas.com/case-study/)

To determine kelp bed extent, density and biomass along the South African West Coast, multiple stakeholders initiated a project to gather more precise orthorectified aerial imagery as the current National Biodiversity Assessment maps were found to be inaccurate in certain areas. These stakeholders included the University of the Western Cape (UWC), the Seaweed Unit of the Department of Fisheries, Forestry and the Environment (DFFE), the kelp industry and two conservation agencies – South African National Parks (SANParks) and CapeNature.

The intention was to survey the 32-km coastline near Gansbaai, a coastal town 160 km south-east of Cape Town. The area covered started approximately 4 km north of the Gansbaai harbour in the west, continuing around the Gansbaai peninsula and ending short of the Pearly Beach residential area in the east.

To capture the extent of the kelp, about 75% of the nearshore ocean had to be included in the survey swathe. The Dyer Island survey area included the kelp beds surrounding the island as well as the kelp beds between the island and the closest shore, an area of roughly 6.5 km².

Project requirements

Project delivery

SAEON was requested to participate in the project and conduct the airborne surveys. SAEON is a science network of people, organisations and, most importantly, observation platforms that perform long-term ecological research in South Africa and its surrounding oceans. SAEON’s Elwandle Coastal Node, based in Gqeberha at the Ocean Sciences Campus of the Nelson Mandela University – and focusing on long-term monitoring and research on South Africa’s coastal zone – was the ideal partner with local knowledge and expertise to deliver this project in the short timeframe required.  

Data collection

SAEON scientists chose LiDAR and the GeoLas ELMAP sensor to collect data for this project.

The GeoLas ELMAP Series 

The GeoLas ELMAP Series is a state-of-the-art mid- to high-range LiDAR sensor designed for integration in manned fixed-wing and helicopter aircraft, gyrocopters, ultralight aircraft and heavy-lift drones. Built to withstand the harshest environments, the ELMAP series offers unparalleled performance and ease of use.

Highlights:

  • Compact all-in-one mapping systems
  • Huge 80° field-of-view
  • Linear uniform scan pattern with parallel scan lines
  • Integrated removable data storage on standard SSDs
  • Waveform digitisation for every pulse

“Our ELMAP-V LiDAR is a perfect fit for our Glasair Sportsman aircraft. We have performed several projects with it and are very impressed with its performance, reliability, ease of use and data quality.” – Sean Bailey, Airborne Remote Sensing Platform Coordinator at SAEON.

Data 

The data captured included detailed imagery and LiDAR data to produce high-resolution orthophotos in GeoTiff format. The data collection flights were meticulously planned to cover the specified areas and achieve the required point densities.

The project was flown with a Glasair Sportsman GS-2 aircraft with a ground speed of 100 knots over a short flight time of only 2,25 hours for the coastline survey (25 strips) and 1,09 hours (three strips) for the Dyer Island survey.

Due to the project parameters and required point densities, the flying altitude for the coastal project was 1 900 ft, and 5 000 ft for Dyer Island.

A total of 282 images for the coastal survey and 143 images for Dyer Island were captured during the flight.

Sean Bailey, Airborne Remote Sensing Platform Coordinator at SAEON

Glasair Sportsman GS-2

Challenges 

The flights were supposed to be conducted over the spring low tide, however, it was difficult to coordinate because of the challenging weather conditions during the airborne campaign. The flight over Dyer Island was particularly challenging since SAEON had to stay within gliding distance of the shore due to their single-engine aircraft. This required SAEON to fly at 5 000 feet ASL, where winds were over 35 knots. Additionally, SAEON could only fly in one direction to keep the ground speed under 100 knots, which affected the lidar resolution, but still allowed them to produce a sufficient DTM.

Adding to the challenges, they had to be based about a 50-minute flight away in Swellendam, as they could not find avgas or hangarage closer to the project site, so the flight time had to be coordinated with the tide.

Data processing

The LiDAR data was used to generate DTMs, which were then used to create orthophotos. The high-resolution imagery was processed to accurately determine the extents, densities and biomass of the kelp beds.

The project successfully delivered the required orthophotos and LiDAR data within the specified timeframe. The high-resolution images and accurate mapping provided by the ELMAP data met the needs of all stakeholders, enabling more precise and reliable kelp bed management and conservation efforts.

RGB image coastal area

DTM coastal area

DTM overlay with intensity

Main take-aways – recap 

  • Technology selection: Choosing the right technology (LiDAR over photogrammetry) is crucial for accurate data collection in challenging environments and terrains.
  • Stakeholder collaboration: Close collaboration between stakeholders and project delivery teams ensures that the data collected meets their specific needs.
  • Efficiency in data processing: Using advanced data processing techniques such as integrating LiDAR-derived DTMs with imagery enhances data quality, accuracy and reliability, which are crucial for conservation and management efforts.

The successful execution of this study demonstrates the efficacy of LiDAR technology in coastal and marine applications, particularly in environments where traditional photogrammetry falls short.

The GeoLas ELMAP system proved to be an excellent choice for this project, providing the accuracy and efficiency needed to meet the diverse requirements of the stakeholders involved.

RGB image Dyer Island

DTM overlay with intensity Dyer Island