The solar radiation balance in buildings has a significant impact on their energy needs, as well as on their potential for buildings-integrated photovoltaics (BIPV) energy production. It also influences the potentials of daylight, its healthiness and sustainability. Solar radiation models for urban environments require the characterization of the obstruction degree to which each point is subjected due to other buildings, topography, vegetation, etc. This characterization is carried out with the parameter known as sky view factor (SVF). In this paper, we check that significant disagreements exist in the literature with respect to the definition of SVF. Most published methods show that SVF admits a geometric interpretation as a ratio of projected sky surface versus the global surface of the sky vault projected on the same system. Nevertheless,the type of projection depends only on the authors' considerations. The geometric comparison,of the methods opens a new way to explain their differences. This paper presents a general mathematical method to obtain projection equations, which allows the interpretation of SVF as a surface ratio. It is also proposed the adequate projection for horizontal surfaces under the hypothesis of angular distribution of diffuse radiance based on Moon-Spencer's model. (C) 2014 Elsevier B.V. All rights reserved.