This article suggests a theory of egocentric semantic reference systems for human observations of affordances that can be used to semantically account for subjective human observations on the web. Based on the perceptual theory of affordances, which suggests that humans perceive the potential actions the environment affords, an egocentric semantic reference frame is established, which is anchored in the observer's specific capabilities for perception and action. The theory is completed with transformations that allow to project values of observed affordances from one user's ordinal reference frame into another user's ordinal reference frame. The potency of the theory to capture the semantics of human observations is demonstrated through the implementation of a full fledged egocentric semantic reference system for the prototypical affordance of hikability that a mountain path affords to a hiker. The prototype uses real user-ratings from a community driven web portal.