Although information modeling techniques and ontologies developments have been intensively used and studied, we believe that clear and logical links between these different concepts are missing. It is even more crucial in 3D city modeling context where standardization processes are less developed. In this paper, we first give an unambiguous view of the interrelations and dependencies between models and ontologies based on a top-level characterization of data, knowledge and information. We then propose an ontology of space aiming at strengthening basis of 3D City models, with a specific application to some objects of the CityGML 3D geo-information standard.