Some subscribers to Marmam may be interested in this paper which describes a
method for measuring range to animals at sea and accurately tracking their
movements. It should find applications in survey, behavioural studies and
Jonathan Gordon (email@example.com)
Measuring the range to animals at sea from boats using photographic and
Journal of Applied Ecology 38 (4), 879-887
Estimating range to objects at sea by eye is notoriously difficult yet there
are many occasions in management and research when accurate measures of
range are required.
A new method is described in which range is calculated from the angle
subtended between the horizon and the waterline of the object measured from
single video or photographic images taken at a known elevation. Possible
errors are explored and practical analysis methods outlined. Images can be
collected and analysis performed using relatively inexpensive standard
In offshore waters, uncertainty in the height of the camera and target
object due to the effects of waves and swell are likely to be the most
significant sources of error. The effects of errors in camera height on
ranges calculated using this method are approximately proportional to the
ratio of the height error to camera height. Simulations indicate that wave-
and swell-induced errors in the height of both object and camera will lead
to range estimates with a standard deviation (SD) of 0.5 ? (error due to
wave height)/(camera height).
Three trials were conducted in which ranges (out to 2000 m) measured using
this method were compared with those determined using alternative methods
(laser range-finding binoculars or non-differential global positioning
system). Mean percentage discrepancies in range measurements between the two
methods varied from 6.4% to 2.6%, while the SD of discrepancies in trials
varied between 6.5% and 4.3%. The independent range measuring methods used
here were not without error, however, and it is suggested that a mean
absolute error of > 2% is an appropriate figure for the video method.
Field tests indicated that the necessary photos or video sequences could be
collected from most types of cetaceans in the field. A variety of
applications for the method during activities such as line transect surveys,
mitigation monitoring and behavioural studies are suggested and discussed.
Key-words : cetacean, line transect, mitigation, photogrammetry,
Present address and correspondence: Jonathan Gordon, Sea Mammal Research
Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife
KY16 8LB, UK (fax 01334 462632; firstname.lastname@example.org).
Received 4 August 2000; revision received 2 January 2000
This archive was generated by hypermail 2b30 : Tue Aug 14 2001 - 13:02:02 EDT