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Knowledge of occurrence and severity of the damage caused by the fungus Cryphonectria parasitica (Murrill) M.E. Barr and forest fires is key to define a management plan of Castanea sativa forest masses in south-western Europe. The main goal of our study is to verify whether there is a concordance between field measurements and measures from orthophotographs acquired by an unmanned flight and to determine the influence of the number of severity levels on it. Accuracy of blight severity level estimate was computed used as ground truth a red-green-blue (RGB) orthophotograph of very high spatial resolution (8 cm) acquired by an Unmanned Aerial Vehicle (UAV). Starting from codes specifically defined for this study, the severity level of 823 chestnut trees was measured on the UAV orthophotograph. At same time, we ground measured the severity level on a sample of 182 chestnut trees using a standard methodology. From these measurements, overall accuracy and Kappa statistic were computed. Our results show that the concordance for 6 severity levels varies between moderate and good, whereas for 4 and 5 levels, varies between moderate and very good (kappa statistic>0.75). The study demonstrates the UAVs usefulness for studying forest damage in South-Western Europe.