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First published online December 14, 2005
Journal of Experimental Biology 209, 128-140 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01970

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Interpolation of animal tracking data in a fluid environment

Yann Tremblay^1,*, Scott A. Shaffer¹, Shannon L. Fowler¹, Carey E. Kuhn¹, Birgitte I. McDonald¹, Michael J. Weise¹, Charle-André Bost², Henri Weimerskirch², Daniel E. Crocker³, Michael E. Goebel⁴ and Daniel P. Costa¹

¹ University of California, Santa Cruz, Long Marine Laboratory, Center for Ocean Health, 100 Shaffer Road, Santa Cruz, CA 95060, USA
² Centre d'Etude Biologiques de Chizé, 79360 Villiers en Bois, France
³ Department of Biology, Sonoma State University, Rohnert Park, CA 94928, USA
⁴ NOAA, National Marine Fisheries, 8604 La Jolla Shores Drive, La Jolla, CA 92038, USA

^* Author for correspondence (e-mail: tremblay{at}biology.ucsc.edu)

Accepted 7 November 2005

Interpolation of geolocation or Argos tracking data is a necessity for habitat use analyses of marine vertebrates. In a fluid marine environment, characterized by curvilinear structures, linearly interpolated track data are not realistic. Based on these two facts, we interpolated tracking data from albatrosses, penguins, boobies, sea lions, fur seals and elephant seals using six mathematical algorithms. Given their popularity in mathematical computing, we chose Bézier, hermite and cubic splines, in addition to a commonly used linear algorithm to interpolate data. Performance of interpolation methods was compared with different temporal resolutions representative of the less-precise geolocation and the more-precise Argos tracking techniques. Parameters from interpolated sub-sampled tracks were compared with those obtained from intact tracks. Average accuracy of the interpolated location was not affected by the interpolation method and was always within the precision of the tracking technique used. However, depending on the species tested, some curvilinear interpolation algorithms produced greater occurrences of more accurate locations, compared with the linear interpolation method. Total track lengths were consistently underestimated but were always more accurate using curvilinear interpolation than linear interpolation. 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. The choice of the curvilinear algorithm does not affect the resulting track dramatically so it should not preclude their use. We thus recommend using curvilinear interpolation techniques because of the more realistic fluid movements of animals. We also provide some guidelines for choosing an algorithm that is most likely to maximize track quality for different types of marine vertebrates.

Key words: tracking, telemetry, Argos, geolocation, GPS, Bézier, cubic, hermite, spline, albatross, penguin, sea lion, fur seal, elephant seal, booby, seabird, marine mammal

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