The use of Global Positioning Systems to record distances in a helicopter line-transect survey

Tiago A. Marques, Magnus Andersen, Signe Christensen-Dalsgaard, Stanislav Belikov, Andrei Boltunov, Oystein Wiig, Stephen T. Buckland, Jon Aars

Research output: Contribution to journalArticlepeer-review

24 Citations (Scopus)


Methods that allow unbiased estimation of animal abundance are increasingly demanded in management and conservation. The use of these methods should respect their assumptions. The need for accurate distance measurements in distance-sampling surveys is stressed. Here we present 2 alternative methods for measuring distance from a line to an object during helicopter surveys: 1) using a Global Positioning System (GPS) unit, with distances measured using appropriate software; and 2) recording declination angles and altitudes, using basic trigonometry to obtain the appropriate distances. These are compared to distances measured by a laser rangefinder (assumed to be true distances). The effect of the different errors on estimated densities is assessed by simulation. The GPS method appeared to be very accurate, while a potential downward bias in estimated density could be present if the inclinometer method is used. We discuss the implication for wildlife studies of using different measurement methods leading to different errors.

Original languageEnglish
Pages (from-to)759-763
Number of pages5
JournalWildlife Society Bulletin
Publication statusPublished - Oct 2006


  • bias
  • distance sampling
  • Global Positioning System
  • GPS
  • helicopter survey
  • inclinometer
  • line transects
  • measuring distances
  • simulation
  • SEA


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