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Comparisons of measured and estimated distances and angles from sightings surveys

Research output: Contribution to journalArticle

Abstract

Photogrammetric systems using video cameras were used to measure radial distances to sightings during the SCANS-II, CODA and SOWER
surveys. These surveys included sightings of a variety of species from harbour porpoise, at distances of a few hundred metres, to large baleen
whales at distances greater than 10km. A total of 910 initial sightings with estimated distances from reticles and measured distances from video,
using 7 × 50 (636) or 25× ‘Big Eye’ (274) binoculars, were compared. Bearings to sightings were also measured from still images. The CVRMSE in
distances varied between 0.19 and 0.33 for reticle binoculars. Comparisons of measured distances to simultaneous sightings by other observers
using naked eye gave a CVRMSE of 0.39 for naked eye estimates. There was a consistent, non-linear pattern in all data sets, of over-estimating close
distances to sightings of surfacing cetaceans and under-estimating those further away. However, this pattern was not evident from the distance
experiments on SOWER to fixed targets which also had a much lower variance (CVRMSE = 0.13). Bearing data from SCANS-II and CODA showed
around 5% of estimates had gross errors greater than 20º that were attributed to mistakes. For the remaining values, RMS errors were in the range
5.7º–7.2º for SCANS-II and CODA and 4.9º for SOWER. Both distance and angle errors will make a substantial contribution to the variance of
abundance estimates and simulated data showed that the observed non-linear nature of distance errors may cause considerable bias even when
linear regressions might suggest little bias. There still remain technological challenges in operating complex electronic systems at sea to measure
distances and bearings, but investment in these methods should be a cost effective way of reducing bias and improving precision of cetacean
abundance estimates.
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Details

Original languageEnglish
Pages (from-to)229-237
Number of pages9
JournalJournal of Cetacean Research and Management
Volume11
Issue number3
Publication statusPublished - May 2010

    Research areas

  • SURVEY-VESSEL, PHOTOGRAMMETRY

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ID: 22715708