Context reasoning is the process of drawing con-clusions from existing context information, and is considered crucial to geospatial interoperability. However, such reasoning still remains a challenge because context may be incomplete or not appro-priate for a specific use. Thus, evaluating and modeling the external quality (fitness-for-use) of context information is important for reasoning about context. In this paper, we propose an ontology-based approach to model the external quality of context information. This approach aims at fine-tuning context reasoning and hence enhancing geospatial interoperability.