![]() ![]() Correcting the measurements for refraction improved their accuracy, eliminating the bias for one ship and reducing it for the other. Refraction had less of an effect than expected for a temperature gradient based on a standard atmosphere, suggesting a mean gradient for the eastern tropical Pacific of –0.02☌ m –1 in the first 10m above the sea surface rather than the standard value of –0.0065☌ m –1. The negative bias in measurements of distances from 4-8km was due to refraction of light and other factors. ![]() Distances beyond about two-thirds of the way to the horizon were not measurable because the angles were too small. Between 4 and 8km (approximately one-third to two-thirds of the distance to the horizon), distances tended to be slightly underestimated, reaching a maximum bias at the most distant targets of 6% for one ship and 16% for the other. ![]() Reticles in 253 binoculars provided unbiased measurements to about a third of the way to the horizon, or from 0-4km for the 10.5m platform heights used for the study. Alternative formulae for calculating distance from optical devices were applied to the reticle measurements and compared to distances measured by radar. Distances were measured under a range of conditions representing the environmental variability in three years of field surveys. The distances to 1,576 targets between 0.3 and 10.4km from two ships were measured using the reticle scale in 253 binoculars during cetacean surveys in the eastern tropical Pacific Ocean. ![]()
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