Participatory sensor networks (PSNs) regards smartphone users as consumers as well as active producers of data. A sensing layer represents a type of data, coming from a given source of data, such as web services, traditional wireless sensor networks, and PSNs. In this work, we show the usefulness and potential of having sensing layers in PSNs. We also show how we can formalize the concept of sensing layers in participatory sensor networks. Furthermore, we demonstrate how to derive and create new applications and services that are not promptly available from different sensing layers, opening up very interesting research opportunities.