Journal Article

Flying high: Sampling savanna vegetation with UAV‐lidar

Singh, Jenia, Boucher, Peter B., Davies, Andrew B., Hockridge, Evan G.

2023

Overview

The flexibility of UAV‐lidar remote sensing offers a myriad of new opportunities for savanna ecology, enabling researchers to measure vegetation structure at a variety of temporal and spatial scales. However, this flexibility also increases the number of customizable variables, such as flight altitude, pattern, and sensor parameters, that, when adjusted, can impact data quality as well as the applicability of a dataset to a specific research interest. To better understand the impacts that UAV flight patterns and sensor parameters have on vegetation metrics, we compared 7 lidar point clouds collected with a Riegl VUX − 1LR over a 300 × 300 m area in the Kruger National Park, South Africa. We varied the altitude (60 m above ground, 100 m, 180 m, and 300 m) and sampling pattern (slowing the flight speed, increasing the overlap between flightlines and flying a crosshatch pattern), and compared a variety of vertical vegetation metrics related to height and fractional cover. Comparing vegetation metrics from acquisitions with different flight patterns and sensor parameters, we found that both flight altitude and pattern had significant impacts on derived structure metrics, with variation in altitude causing the largest impacts. Flying higher resulted in lower point cloud heights, leading to a consistent downward trend in percentile height metrics and fractional cover. The magnitude and direction of these trends also varied depending on the vegetation type sampled (trees, shrubs or grasses), showing that the structure and composition of savanna vegetation can interact with the lidar signal and alter derived metrics. While there were statistically significant differences in metrics among acquisitions, the average differences were often on the order of a few centimetres or less, which shows great promise for future comparison studies. We discuss how these results apply in practice, explaining the potential trade‐offs of flying at higher altitudes and with alternate patterns. We highlight how flight and sensor parameters can be geared toward specific ecological applications and vegetation types, and we explore future opportunities for optimizing UAV‐lidar sampling designs in savannas. A cross‐section of a lidar point cloud for a single tree (left) from a savanna in the Satara region of Kruger National Park, South Africa (visualised with CloudCompare 2.11). Smoothed vertical profiles of fractional canopy cover per 10 cm height bin are plotted for the same tree. These profiles were derived from a series of airborne lidar data collected from 4 different flight altitudes (60 m, 100 m, 180 m, and 300 m above ground) with an unoccupied aerial vehicle (UAV). As flight altitude increases (left‐right), the canopy cover profiles change shape and shift downward, demonstrating that UAV fight and sensor parameters can have a significant impact on lidar measurements of vegetation structure.

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Accuracy / active remote sensing / Airspeed / Altitude / Aviation / Cameras / canopy cover / canopy height / Data collection / Design / Ecological studies / Flexibility / Flight / Flight altitude / Flight characteristics / Forests / Grasslands / Kinematics / Kruger National Park / Lidar / National parks / Parameters / Remote sensing / Remote sensors / Sampling / Sampling designs / savanna / Savannahs / Scanners / Sensors / Shrubs / Statistical analysis / UAS / UAV / Unmanned aerial vehicles / unoccupied aerial systems / unoccupied aerial vehicles / Vegetation / vegetation survey / Vegetation type / Vertical flight