In the field of remote sensing applications, vegetation indices(VI) have been developed for qualitatively and quantitatively evaluating vegetative covers using spectral measurements. The spectral response of vegetated areas presents a complex mixture of vegetation, soil brightness, environmental effects,shadow, soil color and moisture. Moreover, the VI is affected by spatial-temporal variations of the atmosphere. Overforty vegetaion indices have been developed during the past two decades in order to enhance vegetation response and minimize the effects of the factors described above. Most of the vegetation indices were summarized, discussed, analysed about their applicability and limitations, and simply classificated. Vegetation indices are quantitative measurements indicating the vigor of vegetation. They show better sensitivity than individual spectral bands for green vegetation detection. Their usefulness lies as an aid to remote sensing image interpretation, the detection of land use changes, the evaluation of vegetative cover density, forestry, crop discrimination and crop prediction.
In general, it can be observed that vegetation indices do not have a standard universal value, research having ofen shown different results. The atmosphere, sensor calibration, sensor viewing conditions, solar illumination geometry, soil moisture, color and brightness seriously affect vegetaion indices. Moreover, in a heterogeneous environment, where there is a mixture of vegetation and other ground elements in the pixels, the study of vegetation indices becomes more complex. However, the choice of a vegetation index as opposed to another, for what ever application, is quit delicate to make. Each environment has its own characteristics and each index is an indicator of green vegetation in its own right. As hyperspectral remote sensing technology (such as AVIRIS) and thermal infrared multi-spectral remote sensing technology (such as ASTER) goes on, many VI will be developed.