Impact of camera focal length and sUAS flying altitude on spatial crop canopy temperature evaluation

Abstract

Thermal image quality is critical to accurately quantify spatial and temporal growth and stress patterns of field crops. Image data quality from a thermal sensor can be impacted by several factors including environment, flying altitude, and camera focal length. Often times the thermal sensor selection is based upon price or ease of availability for research. Metrics are available to select the flight altitude based on thermal sensor focal length for desired ground resolution, however, no study has been conducted to provide the relative difference in image data, quality and efficiency of generating a thermal orthomosaic. Therefore, this study was conducted with the goal to compare the accuracy of canopy temperature quantification and assess the quality of thermal orthomosaic when using a thermal sensor of different focal lengths and image acquisition at varying flying altitudes of an sUAS. Three thermal infrared cameras were selected with focal lengths of 9 mm, 13 mm, and 19 mm. All three cameras were flown at altitudes of 20 m, 50 m, and 80 m, to collect aerial imagery of a 7,000 m2 soybean field. The cameras were mounted on a rotary quadcopter. All flights were conducted at 3 m/s flying speed and 1 s shutter trigger interval. A ground reference system comprising of a panel and water bath system with measured actual temperature provided ground truth data for thermometric transformations. Imagery data were compared to assess differences in the number of images collected, percentage overlap required for 1 s shutter trigger interval, quality of orthomosaic and accuracy of canopy temperatures. Results showed that 13 mm focal length and 50 m altitude resulted in a finer resolution orthomosaic which can provide robust and accurate information on canopy temperature. The selection of such a system of camera lens angle and altitude can provide accurate, reliable and rapid canopy temperature quantification.