1. A method for generating a model of a physical entity using one or more hierarchically organized multi-resolutional data trees T, each data tree comprising a plurality of nodes N k , each node having a grid with a fixed number of points to which data is associated, the method comprising the steps of: creating a geometry using one or more data trees; and creating an associated overlay using one or more data trees, wherein at least one of the geometry and overlay comprises a formula to be applied to data within one or more data trees.