For example,acoustic tracking of both oceanographic floats( Fratantoni and Richardson,1999) and any seabird observed for a short time at sea classically shows a sinuous path ( Alerstam et al.,1993 Weimerskirch et al.,2000). Fluid media are kingdoms of curves, being described by flows, vortices, turbulences and gradients( Vogel, 1994). However, straight lines are not consistent with fluid dynamics in which subjects moving in a fluid environment (air or water) probably do not follow straight lines. The advantages of linear interpolation are its simplicity and that it represents the most conservative path an animal transits between two consecutive locations. Filtering of tracking data (by removing unlikely locations) addresses the problem of location inaccuracy and has received more attention than interpolation( Austin et al., 2003 McConnell et al., 1992 Sibert et al., 2003).Įither by choice or lack of an alternative, most authors represent their tracking data as straight lines between recorded points and do not interpolate their data ( Block et al., 2005 Folkow et al., 2004 Pütz et al., 2000). These two characteristics determine track quality and generally imply two levels of post processing: filtering and interpolating. These techniques differ with respect to two fundamental characteristics: (1) the location accuracy and (2) the frequency at which locations are obtained. Satellite telemetry (using the Argos system), geolocation (by recording day length), and GPS (Global Positioning System) telemetry are the main tracking techniques, with satellite telemetry being used most commonly. Such devices can provide information on the geo-position of the tracked animal for a given time, as well as other behavioral, physiological or environmental information( Kooyman et al., 1992 Weimerskirch et al., 1997 Wilson et al., 1995). Our understanding of what marine vertebrates do when they are out of sight thus relies almost exclusively on recording and/or transmitting electronic devices. We also provide some guidelines for choosing an algorithm that is most likely to maximize track quality for different types of marine vertebrates.Įcological studies of marine vertebrates have proven challenging due to our inability to observe individuals for long periods. We thus recommend using curvilinear interpolation techniques because of the more realistic fluid movements of animals. The choice of the curvilinear algorithm does not affect the resulting track dramatically so it should not preclude their use. Curvilinear algorithms are safe to use because accuracy, shape and length of the tracks are either not different or are slightly enhanced and because analyses always remain conservative. Total track lengths were consistently underestimated but were always more accurate using curvilinear interpolation than linear interpolation. However, depending on the species tested, some curvilinear interpolation algorithms produced greater occurrences of more accurate locations, compared with the linear interpolation method. Average accuracy of the interpolated location was not affected by the interpolation method and was always within the precision of the tracking technique used. Parameters from interpolated sub-sampled tracks were compared with those obtained from intact tracks. Performance of interpolation methods was compared with different temporal resolutions representative of the less-precise geolocation and the more-precise Argos tracking techniques. Given their popularity in mathematical computing,we chose Bézier, hermite and cubic splines, in addition to a commonly used linear algorithm to interpolate data. Based on these two facts, we interpolated tracking data from albatrosses, penguins, boobies, sea lions, fur seals and elephant seals using six mathematical algorithms. In a fluid marine environment,characterized by curvilinear structures, linearly interpolated track data are not realistic. Interpolation of geolocation or Argos tracking data is a necessity for habitat use analyses of marine vertebrates.
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