Evaluating Alternative Flight Plans in Thermal Drone Wildlife Surveys—Simulation Study

Julia Witczuk,Stanisław Pagacz

doi:10.3390/rs13061102

Julia Witczuk, Stanisław Pagacz

Open Access

https://doi.org/10.3390/rs13061102

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Abstract

The rapidly developing technology of unmanned aerial vehicles (drones) extends to the availability of aerial surveys for wildlife research and management. However, regulations limiting drone operations to visual line of sight (VLOS) seriously affect the design of surveys, as flight paths must be concentrated within small sampling blocks. Such a design is inferior to spatially unrestricted randomized designs available if operations beyond visual line of sight (BVLOS) are allowed. We used computer simulations to assess whether the VLOS rule affects the accuracy and precision of wildlife density estimates derived from drone collected data. We tested two alternative flight plans (VLOS vs. BVLOS) in simulated surveys of low-, medium- and high-density populations of a hypothetical ungulate species with three levels of effort (one to three repetitions). The population density was estimated using the ratio estimate and distance sampling method. The observed differences in the accuracy and precision of estimates from the VLOS and BVLOS surveys were relatively small and negligible. Only in the case of the low-density population (2 ind./100 ha) surveyed once was the VLOS design inferior to BVLOS, delivering biased and less precise estimates. These results show that while the VLOS regulations complicate survey logistics and interfere with random survey design, the quality of derived estimates does not have to be compromised. We advise testing alternative survey variants with the aid of computer simulations to achieve reliable estimates while minimizing survey costs.

Highlights

Accepted: 8 March 2021Estimating population size is one of the fundamental tasks in wildlife management and conservation
The surveys of the low-density population with low effort—did the visual line of sight (VLOS) design appear to be significantly inferior to beyond the visual line of sight (BVLOS), showing moderate bias
Wildlife surveys with the use of drones are usually constrained by regulations limiting drone operations to visual line of sight

Summary

Introduction

Accepted: 8 March 2021Estimating population size is one of the fundamental tasks in wildlife management and conservation. Since ungulate populations often occupy large, mostly forested areas, surveying them is logistically difficult and costly. The currently used ground-based survey methods, such as drive counts or snow tracking are acknowledged to be unreliable and inefficient [2]. Alternative methods such as aerial surveys are more effective; over the forests, their use remains limited, as camouflage coloration of ungulate coats impedes their detection on the cluttered background of the forest floor. In areas where ungulates are active mostly during dusk and dawn [3,4,5], the applicability of aerial surveys is further reduced; in times of poor visibility, especially at lower altitudes, manned aircraft flights are challenging and dangerous

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