Measuring the range to animals at sea from boats using photographic and video images

Abstract

Summary 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 equipment. 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.