Investigating modelled and observed Terra/MODIS 500-m reflectance data for viewing and illumination effects Original Research Article

MODIS data are needed for the development of a light-use efficiency model over Swedish forests. Sweden stretches between Lat. 55N. and Lat. 69N. and MODIS reflectance data are recorded at vastly different sun and viewing angles. We have investigated modelled and observed red and near-infrared (NIR) MODIS data, and the normalised difference vegetation index (NDVI), in order to enable the development of a processing scheme that can be used for correcting MODIS data for directional effects. Directional reflectances have been simulated with the Kuusk–Nilson forest reflectance model, based on data from the Swedish National Forest Inventory. Modelled BRDF data are stored in a look-up table (LUT) and the possibility of finding a match with the observed data is tested. The analysis shows that matching may be based on normalised data rather than absolute values. Directional effects in the modelled data differ between sparse, medium and dense forest stands. The matching of observed MODIS data to the model data stored in the LUT is severely hampered by the scatter in the MODIS data. Reducing this scatter, e.g., by an improved cloud screening and improved atmospheric correction, will be necessary for the development of a reliable correction scheme.