All software engineering starts with assumptions about the intended purpose, basic functionality, potential users, and their requirements. Developers agree on a shared terminology and workflow, for instance using UML. In fact, they agree on ontological commitments – on an application dependent model of the world. Such commitments, however, are most often driven by business needs and are based on the restrictions of the used platform, e.g., smartphones. What most developers overlook is that (due to the ubiquity of mobile devices and apps) their ontological decisions also affect how users conceptualize geographic space. The role of ontologies in software engineering has been acknowledged and studied before. In this work, based on a Volunteered Geographic Services application for Android smartphones, we discuss how specific design choices affect the user’s behavior and knowledge. We present first ideas on how such choices can be documented and made explicit.