Distance sight-resight sampling has particular relevance to aerial surveys, in which height above ground and aircraft speed make the critical assumption of certain detection on the track-line unrealistic. Recent developments in distance sight-resight theory have left practical issues related to data collection as the major impediment to widespread use of distance sight-resight sampling in aerial surveys. We describe and evaluate a system to automatically log, store, and process data from distance sight-resight aerial surveys. The system has a primary digital system and a secondary audio system. The digital system comprises a sighting 'gun' and small keypad for each observer, a global positioning system (GPS) receiver, and an altimeter interface, all linked to a central laptop computer. The gun is used to record time and angle of declination from the horizon of sighted groups of animals as they pass the aircraft. The keypad is used to record information on species and group size. The altimeter interface records altitude from the aircraft's radar altimeter, and the GPS receiver provides location data at user-definable intervals. We wrote software to import data into a database and convert it into a form appropriate for distance sight-resight analyses. Perpendicular distance of sighted groups of animals from the flight path is calculated from altitude and angle of declination. Time, angle of declination, species, and group size of sightings by independent observers on the same side of the aircraft are used as criteria to classify single and duplicate sightings, allowing testing of the critical distance sampling assumption (g(0)=1) and estimation of g(0) if that assumption fails. An audio system comprising headphones for each observer and a 4-track tape recorder allows recording of data that are difficult to accommodate in the digital system and provides a backup to the digital system. We evaluated the system by conducting experimental surveys and reviewing results from actual surveys.
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An adaptive management approach to controlling suburban deer