Geolocation by light: accuracy and precision affected by environmental factors

Research output: Contribution to journalArticle

Abstract

1. Geolocation by light allows for tracking animal movements, based on measurements of light intensity over time by a data-logging device (geolocator). Recent developments of ultra-light devices (<2 g) broadened the range of target species and boosted the number of studies using geolocators. However, an inherent problem of geolocators is that any factor or process that changes the natural light intensity pattern also affects the positions calculated from these light patterns. Although the most important factors have been identified, estimation of their effect on the accuracy and precision of positions estimated has been lacking but is very important for the analyses and interpretation of geolocator data. 2. The threshold method is mainly used to derive positions by defining sunrise and sunset times from the light intensity pattern for each recorded day. This method requires calibration: a predefined sun elevation angle for estimating latitude by fitting the recorded day/night lengths to theoretical values across latitudes. Therewith, almost constant shading can be corrected for by finding the appropriate sun elevation angle. 3. Weather, topography and vegetation are the most important factors that influence light intensities. We demonstrated their effect on the measurement of day/night length, time of solar midnight/noon and the resulting position estimates using light measurements from stationary geolocators at known places and from geolocators mounted on birds. Furthermore, we investigated the influence of different calibration methods on the accuracy of the latitudinal positions. 4. All three environmental factors can influence the light intensity pattern significantly. Weather and an animals behaviour result in increased noise in positioning, whereas topography and vegetation result in systematic shading and biased positions. Calibration can significantly shift the estimated latitudes and potentially increase the accuracy, but detailed knowledge about the particular confounding factors and the behaviour of the studied animal is crucial for the choice of the most appropriate calibration method.

Details

Authors
Organisations
External organisations
  • Natural History Museum of Denmark
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Biological Sciences

Keywords

  • animal movement, calibration methods, geolocation, migration, tracking
Original languageEnglish
Pages (from-to)603-612
JournalMethods in Ecology and Evolution
Volume3
Issue number3
Publication statusPublished - 2012
Publication categoryResearch
Peer-reviewedYes