Marine Ornithology

To determine strip boundaries for ship-based surveys, seabird biologists commonly use the handheld rangefinder developed by Heinemann (1981). However, this tool's reliance upon the horizon as a plane of reference limits its use in confined channels or similarly enclosed areas. We have developed a rangefinder that uses a level, bubble-sight hole combination to establish a horizontal plane at the observer's eye level, from which range lines can be measured on the tool. Here we present its design and best practices to minimize bias and variance. Sensitivity analysis demonstrated that, of all variables tested, error in holding angle contributes most to error in strip width determination, followed in distant second by error in arm length. The performance of this rangefinder will be limited by the combination of platform height and target strip width. Our design, simulations and trials of the rangefinder allow us to predict error in strip width (e.g. coefficient of variation of 0.22 is expected for target strip width of 150 m from a platform height of 10 m), which can then be incorporated into strip-transect analyses to produce more transparent results. We propose the concept of a “zone of uncertainty” around target strip widths for propagating rangefinder performance metrics into the overall variance of a density estimate. For best performance, we recommend the use of 3D printing in rangefinder construction, extreme care in marking range lines on the tool, and field training and calibration sessions before use during formal surveys.