An open-source method for producing reliable water temperature maps for ecological applications using non-radiometric sensors

The number of thermal sensors mounted on Unmanned Aerial Vehicles (UAVs) has increased considerably in recent years, with several of those currently available on the market being very attractive due to their low cost and ease of use. UAV platforms coupled with non-radiometric thermal sensors tend to be least expensive, but few studies have evaluated the reliability of data they produce or developed workflows to improve data accuracy. Here we present a complete method for obtaining accurate and precise water temperature data from non-radiometric sensors, including bias correction, calibration, image stitching and wetted area extraction. We illustrate the utility of the method through its application to the assessment of thermal conditions in two tropical river reaches. The whole procedure is implemented in the R language, and we provide a tutorial and customizable functions. Validation tests indicate that our method produces water temperature estimates that are comparable to those from more expensive sensors (mean absolute error <0.5 °C) and so can be used to answer ecological questions with great confidence. The method dramatically increases the accessibility of thermal imaging in ecology by providing accurate data that is cost-effective and not computationally intensive, and for calibration and validation relies on only small and lightweight equipment that can transported and used in remote locations.