In this paper we introduce mechanisms for automated mapping of urban areas that provide a virtual sensor abstraction to the applications. We envision a participatory system that exploits widely available devices as mobile phones to cooperatively read environmental conditions as air quality or noise pollution, and map these measurements to stationary virtual sensors. We propose spatial and temporal coverage metrics for measuring the quality of acquired sensor data that reflect the conditions of urban areas and the uncontrolled movement of nodes. To achieve quality requirements and efficiency in terms of energy consumption, this paper presents two algorithms for coordinating sensing. The first is based on a central control instance, which assigns sensing tasks to mobile nodes based on movement predictions. The second algorithm is based on coordination of mobile nodes in an ad-hoc network. By extensive simulations, we show that these algorithms achieve a high quality of readings, which is about 95% of the maximum possible. Moreover, the algorithms achieve a very high energy efficiency allowing for drastic savings compared to uncoordinated sensing.