To investigate the basic problems of granular computing, such as granulation, computing with granulars and so on, in a more general setting, a covering model of granular computing is introduced in this paper by relaxing the three conditions of equivalence relations, which generalizes the existing models. Under this model, Zoom-in and Zoom-out operators are defined, respectively. Different combinations of Zoom-in and Zoom-out operators form different rough approximations of the universe of discourse and granulated universe of discourse. This paper studies their propertis and establishes their relationship with topological space and Galois connection.