Geolocation by light: accuracy and precision affected by environmental factors

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Geolocation by light: accuracy and precision affected by environmental factors. / Lisovski, Simeon; Hewson, Chris M.; Klaassen, Raymond; Korner-Nievergelt, Fraenzi; Willemoes Kristensen, Mikkel; Hahn, Steffen.

In: Methods in Ecology and Evolution, Vol. 3, No. 3, 2012, p. 603-612.

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Lisovski, Simeon ; Hewson, Chris M. ; Klaassen, Raymond ; Korner-Nievergelt, Fraenzi ; Willemoes Kristensen, Mikkel ; Hahn, Steffen. / Geolocation by light: accuracy and precision affected by environmental factors. In: Methods in Ecology and Evolution. 2012 ; Vol. 3, No. 3. pp. 603-612.

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TY - JOUR

T1 - Geolocation by light: accuracy and precision affected by environmental factors

AU - Lisovski, Simeon

AU - Hewson, Chris M.

AU - Klaassen, Raymond

AU - Korner-Nievergelt, Fraenzi

AU - Willemoes Kristensen, Mikkel

AU - Hahn, Steffen

PY - 2012

Y1 - 2012

N2 - 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.

AB - 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.

KW - animal movement

KW - calibration methods

KW - geolocation

KW - migration

KW - tracking

U2 - 10.1111/j.2041-210X.2012.00185.x

DO - 10.1111/j.2041-210X.2012.00185.x

M3 - Article

VL - 3

SP - 603

EP - 612

JO - Methods in Ecology and Evolution

T2 - Methods in Ecology and Evolution

JF - Methods in Ecology and Evolution

SN - 2041-210X

IS - 3

ER -