Effect of spatial resolution on thermal and near-infrared sensing of canopies

Satellite observations of agricultural and other plant canopies in the thermal and near iR regime have generally been at spatial scales of tens to hundreds of meters. Advances in sensor technology will extend our capabilities for IR measurements from space to yield improved spatial resolutions. We explore the variability in brightness temperature and the covariation of the normalized difference vegetation index (NDVI) with brightness temperature as a function of viewing geometry and changing spatial resolution. Using 3-D models for both canopy reflectance and thermal infrared exitance, we employ a theoretical analysis for an agricultural scene. The directional viewing effects and correlation between the NDVI and brightness temperature are found to be scale independent and in agreement with experiment. Directional anisotropy in brightness temperature and NDVI are calculated to be less than 7 to 12% respectively for zenith view angles less than 30 deg, but range up to 22 to 40% for zenith view angles of 60 deg. Analysis of variation in local standard deviations with spatial resolution shows a maximum peak corresponding to crop row spacing with rapid fall-off at larger scales.