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This work was conducted with the purpose of validating the hotspots utilized in fire monitoring. The burnt areas observed in six conservation units situated in north of the State of Minas Gerais were mapped in the period of September 3rd to October 5th, 2008, using semi-automatic segmentation of LandSat 5 TM images. A total of 190 burnt areas were mapped and the detections were verified using the satellites hotspots generated by the Brazilian National Space Research Institute - INPE. The burnt areas were also analyzed by size class in order to verify the hypothesis that the smallest areas are less liable to detection. The analysis of the hotspots distance to the limits of the burnt areas was done using buffers, with an increase of 1.00 km in every class, up to the limit of 9.00 km. Considering the investigated burnt areas, about 26.00% were detected, demonstrating limitations of the system to detect burnings smaller than 100.00 ha. In spite of that limitation, a great portion of the impacted area was detected, reaching a total of hits of about 71.00%. The results of location errors were considered satisfactory, considering the technical limitations of the spatial resolution of the utilized sensors. This information may subside the technological advancement of orbital monitoring.