Abstract. As an important factor of VGI quality, this paper focuses on uncertainty arisen in assigning tags to features by VGI users. The VGI portals ask their users to assign (or tag) one or more data types to features, from a set of pre-defined types, whose meanings may be vague for the user, or distinctions between some of them are not clear, i.e. depend on the users’ semantics. This research believes such uncertainties are the results of perceptual issues arising in serial communication between the system and the user. We categorize the problem, and then utilize semantic modelling to reduce such uncertainties. A hierarchy of feature types is produced. At each step, users are asked a simple question with clear distinct answers, which gradually directs the user to the right type. We will describe the approach and present the initial results for the hierarchy produced for major linear features of OpenStreetMap.