Land Search and Rescue Probability of Detection: New sweep widths values, correction factors, models, and detection model validation.

Robert J Koester PhD, FRGS School of the Environment, Geography and Geosciences, University of Portsmouth dbS Productions UK, USA



Search theory allows for correction factors to account for conditions (such as night) that affect the sweep width value. Search theory also predicts that the coverage is proportional to the probability of detection by either the inverse cube curve or exponential function (random search). The objective is to determine the correction factors from night searching and validate the coverage curves.

An Effective Sweep Width experiment was conducted with the same medium visibility adult-sized targets during both the day and the night in a temperate forest. In addition, high and low visibility clue-sized objects were placed directly on the trail. We found an effective sweep width of 64 meters during the daytime and 22 meters at night for a correction factor of 0.34 for the adult-sized targets. Both high (100% vs 94%) and low (83% vs 43%) visibility clues were more detectable during the day versus night (P<0.001).

Searchers with dim flashlights (<200 lux at one meter) resulted in an additional correction factor of 0.5. The probability of detection versus coverage plots of both day and night experiments fell between the inverse cube and exponential curves. This single experiment for only one visibility class of search target showed that visual searching is significantly degraded by searching at night. The daytime coverage suggests the inverse cube model while the night coverage suggests an interim result. The use of effective sweep width, correction factors, and validated coverage curves can lead to more accurate assessments of the probability of detection.

Keywords: decision support systems, search theory, search and rescue, model validation, correction factor.

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