NDVI (Normalized Difference Vegetation Index) is useful measure of live green vegetation obtained with remotely sensed data. For its calculation two images concerning reflectance of the scene in red and infrared bands are necessary. Results can be classified to obtain land cover map distinguishing three classes: waters (oceans, seas, lakes, rivers, …), soils (rocks, buildings, roads, …) vegetation (forests, grasses, orchards, …). Landsat 7 ETM+ images (band 3 and band 4) can be used for NDVI calculation, but if data have been formatted to fit in 8-bit numbers (ranges from 0-255), transformation is necessary to obtain reflectance values. One of the solution is to consider “top of atmosphere” (TOA) measurements: solar radiation incident on the satellite sensor is derived by digital numbers. The aim of this paper is to demonstrate the advantages of this transformation, even if it doesn’t supply the effective reflectance at soil: Landsat ETM+ red and near-infrared images of Sicily were considered and NDVI was calculated in two different ways, with and without TOA transformation of original data. Results were compared to remarks better performance of the first approach.
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