As well as spatial, temporal and spectral scales, which are more related to the act of making observations, it is equally important to consider different levels of spatial aggregation in various Earth science processes. These aggregation levels are mostly levels of spatial aggregation ordered hierarchically. For instance, a forest consists of trees, which have trunks, branches and twigs, on which one finds leaves or needles, and so on. Similarly, an agricultural area may be described in terms of parcels, lots, cropping fields, individual plants, stems and single leaves.