Directional effects on scene complexity in oblique thermal imagery and photographs of a deciduous forest

This paper presents an examination of thermal IR images and photographs of a mixed-specie deciduous forest in eastern Tennessee. Changes of scene complexity with changes of depression angle and azimuth relative to the sun are examined. Ground-based temperature measurements and canopy structure information are used to support the interpretation of view and solar geometry effects on thermal imagery. The green band of digitized oblique photographs from three azimuths are compared to thermal IR images obtained at similar view directions. Thermal IR scenes of the forest are most complex at small phase angles (angle between vectors to the sensor and to the sun) where photographic images were least complex. At these angles, sunlit subcanopy (nonfoliage) components are visible and much warmer than the leaves. At other directions, visible subcanopy materials are more shaded, and their temperatures are similar to leaf temperature. As view azimuth becomes more aligned with the sun, the transition to the more complex and warmer images is rapid. For visible light in this forest, scene complexity is primarily the result of crown illumination and shadowing. Viewing of shadows is minimized at small phase angles so the uniformly illuminated canopy appears simple.