Comparing Drones and Ground Tools for Shoreline Ocean Tracking

Jenn Hoskins
28th June, 2025

Comparing Drones and Ground Tools for Shoreline Ocean Tracking

Drone imagery showing a school of fish (a) being herded (b) and subsequently hunted (c) by a solitary harbour porpoise (Phocoena phocoena) demonstrates the superior capacity of aerial systems to detect complex underwater behaviors that remain unavailable to land-based theodolite operators.

Image adapted from: Dinkel et al. / CC BY (Source)

Key Findings

  • The study in the western Baltic Sea found that land-based theodolites are highly effective for initially spotting harbor porpoises and precisely tracking their surfacing times
  • Drones, while harder to deploy consistently, provide superior detail for counting porpoise group sizes and observing complex behaviors, including those underwater
  • Ultimately, using both theodolites and drones together offers the most comprehensive way to monitor elusive marine mammals like porpoises, combining broad detection with fine-scale insights
Monitoring small marine mammals like the harbor porpoise is notoriously challenging. These animals are often elusive, spend limited time at the surface, and can be found in low-density areas. Understanding their movements, behavior, and population sizes is crucial for conservation, but traditional observation methods face significant hurdles. This difficulty has led researchers to explore various techniques, from visual surveys to advanced technologies, to gain more accurate insights into these creatures. A recent study[1] by the Thünen Institute of Baltic Sea Fisheries addressed this challenge by comparing two key instruments for observing harbor porpoises (Phocoena phocoena) in coastal areas: theodolites and drones. The research aimed to assess their effectiveness in gathering information on porpoise location, behavior, and group size. The study was conducted in the western Baltic Sea, where both methods were used simultaneously during a field campaign in 2022. Theodolites, which are precision optical instruments used by land-based observers to measure horizontal and vertical angles, allowed researchers to pinpoint the surfacing positions of porpoises. Drones, on the other hand, provided an aerial perspective, capturing video footage that could be later analyzed. The findings revealed that each method offered distinct advantages. Theodolite observers were able to detect porpoise surfacing positions in a high percentage of sightings (80.5%). This highlights their reliability for initial detection and tracking. However, the study noted that while drones collected data during 50.7% of total sightings detected by the naked eye, the quality of drone footage was poor in a significant portion (47.3%) of these instances. Despite this, when good quality drone footage was available (75 hours and 36 minutes), cetacean appearance was confirmed for a substantial duration (16 hours and 55 minutes, or 22.4% of the good quality footage). A key finding was that determining group size was significantly more precise using drone footage. This is likely due to the overhead view provided by drones, which allows for a clearer count of individuals within a group, especially when animals are close together or partially obscured from a ground-based view. In contrast, theodolites proved more accurate in pinpointing the exact start and end times of a sighting, which is crucial for tracking individual movements over time. The accuracy of locations recorded by both methods was modeled by comparing the coordinates of the same surfacing porpoise. This "distance between theodolite and drone coordinates" (Dt-d) varied significantly based on the "point quality," referring to the clarity and certainty of the observation. Interestingly, environmental factors like sea state (the roughness of the sea) and the distance between the porpoise and the theodolite observer did not significantly influence this positional accuracy. This research builds upon previous efforts to improve marine mammal monitoring. For example, earlier work has shown the potential of passive acoustic monitoring (PAM) as an alternative to traditional visual surveys, especially for species like harbor porpoises in low-density areas[2]. That study demonstrated how acoustic dataloggers could provide robust density estimates (calculations of how many animals are in a given area), complementing or even replacing visual methods where they are impractical. While the current study focuses on refining visual techniques, it acknowledges the inherent difficulties in monitoring these animals, a problem that acoustic methods also seek to address. The use of drones in the current study also aligns with advancements seen in other marine mammal research. Drones have proven invaluable for observing the behavior of fast-moving, large groups of marine mammals that are challenging to study with traditional tagging methods[3]. For instance, drones have been integrated with acoustic recorders and visual observations to quantify how noise, such as military sonar, affects the behavior of dolphins. Similarly, the current study leverages drones for detailed behavioral and group size observations, echoing the utility demonstrated in these earlier contexts. Furthermore, the application of land-based observation techniques, specifically theodolites, in the current study resonates with prior research that used similar methods to understand the fine-scale movement patterns of marine mammals[4]. For example, theodolite data has been quantitatively analyzed to identify foraging strategies and important feeding sites for bottlenose dolphins by modeling their movements and identifying areas of intensive search effort. The current study's use of theodolites to track movements and determine sighting durations contributes to this broader understanding of how land-based visual tools can reveal crucial aspects of marine mammal ecology. Ultimately, the study concludes that theodolites and drones are complementary tools. When used simultaneously, they provide a more comprehensive dataset: theodolites offer reliable initial detection and precise timing of sightings, while drones excel at detailed observations of group size and fine-scale behavior. This integrated methodology offers a robust way forward for land-based observation studies of harbor porpoises and other marine mammals, enhancing our ability to monitor these important populations.

WildlifeBiotechMarine Biology

References

Main Study

1) Performance of theodolites versus drones in land-based studies of marine mammals

Published 25th June, 2025

https://doi.org/10.1038/s41598-025-06978-8

Related Studies

2) From echolocation clicks to animal density--acoustic sampling of harbor porpoises with static dataloggers.

https://doi.org/10.1121/1.3662070

3) Behavioural responses of common dolphins to naval sonar.

https://doi.org/10.1098/rsos.240650

4) Quantitative analysis of bottlenose dolphin movement patterns and their relationship with foraging.

Journal: The Journal of animal ecology, Issue: Vol 75, Issue 2, Mar 2006


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