Decision rules for determining terrestrial movement and the consequences for filtering high-resolution global positioning system tracks: a case study using the African lion (Panthera leo).

Richard M. Gunner,Pauli Viljoen,Delmar Diesel,O. Louis van Schalkwyk,Phil Hopkins,Rory P. Wilson,D. Michael Scantlebury,Mads F. Bertelsen,Mark D. Holton,Sam Ferreira,Craig J. Tambling,Nikki J. Marks,Martin C. van Rooyen,Nigel C. Bennett,Stephen H. Bell,Carlos M. Duarte,Angela Bruns,Danny Govender,Aoife Göppert

doi:10.1098/rsif.2021.0692

Abstract

The combined use of global positioning system (GPS) technology and motion sensors within the discipline of movement ecology has increased over recent years. This is particularly the case for instrumented wildlife, with many studies now opting to record parameters at high (infra-second) sampling frequencies. However, the detail with which GPS loggers can elucidate fine-scale movement depends on the precision and accuracy of fixes, with accuracy being affected by signal reception. We hypothesized that animal behaviour was the main factor affecting fix inaccuracy, with inherent GPS positional noise (jitter) being most apparent during GPS fixes for non-moving locations, thereby producing disproportionate error during rest periods. A movement-verified filtering (MVF) protocol was constructed to compare GPS-derived speed data with dynamic body acceleration, to provide a computationally quick method for identifying genuine travelling movement. This method was tested on 11 free-ranging lions (Panthera leo) fitted with collar-mounted GPS units and tri-axial motion sensors recording at 1 and 40 Hz, respectively. The findings support the hypothesis and show that distance moved estimates were, on average, overestimated by greater than 80% prior to GPS screening. We present the conceptual and mathematical protocols for screening fix inaccuracy within high-resolution GPS datasets and demonstrate the importance that MVF has for avoiding inaccurate and biased estimates of movement.

Highlights

A popular method to determine terrestrial animal movement uses global positioning system (GPS) technology, which enables long-term continuous spatial monitoring of wild animals without disturbing them
Discrepancies between GPS speed and Vectorial dynamic body acceleration (VeDBA) were associated with location error, with the movement-verified filtering (MVF) approach highlighting that the position of the collar depended on the animal’s behaviour and that this was a prime modulator of GPS performance
Movement-defined thresholds can be modelled according to the focal species in question, while further differences between motion sensor and GPS derivatives can be incorporated into this MVF foundation to resolve fix inaccuracy during movement

Summary

Introduction

A popular method to determine terrestrial animal movement uses global positioning system (GPS) technology, which enables long-term continuous spatial monitoring of wild animals without disturbing them (for reviews see [1,2,3,4,5]) This approach has led to broad applications, including examination of home ranges [6,7], migratory routes [8,9,10], habitat use [11,12], resource allocation [13,14], activity budgets [15,16,17] as well as social interactions [18]. Species-specific resampling strategies and correction factors can go some way to redressing this (see [26,27,28,29])

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