Use of Handheld Thermal Imager Data for Airborne Mapping of Fire Radiative Power and Energy and Flame Front Rate of Spread

Infrared (IR) remote sensing is increasingly used in studies of vegetation fire behavior, and high spatiotemporal resolution investigations often require data to be collected from airborne platforms, for example, standard helicopters. This paper aims to extend the range of conditions under which low-cost “handheld” thermal imaging cameras can be employed in such studies, particularly by enabling the effective and efficient geometric correction of thermal imagery collected from such devices, even when viewing far off-nadir (e.g., out of a side door or window). The approach is based on the automated detection of a set of fixed thermal “ground control points,” coupled with the use of a linear transformation matrix for warping the raw IR imagery to a fixed coordinate system. The output set of geometrically corrected brightness temperature and radiance images can be used to derive fire radiative power (FRP) and flame front rate of spread (ROS). We demonstrate and test our IR image processing methods on a series of case study fires, ranging from a small-scale laboratory to a 945-m² outdoor experimental burn. We compare mapped information on FRP obtained from simultaneous nadir and off-nadir views, where we find differences that are in part controlled by flame structure and/or view angle. In the large open fire case, we compare the mapped fire radiative energy and ROS to simultaneously acquired aerial photography that provides the position of fuel and flames in high detail, and we demonstrate how these data sets can be used to explore various aspects of fire behavior.