This book constitutes the proceedings of the Third International Conference on GeoSensor Networks, GSN 2009, held in Oxford, UK, on July 13-14, 2009. The 15 contributions presented together with 2 invited papers were carefully reviewed and selected from 32 submissions. This volume includes papers covering a variety of topics, ranging from sensing, routing and in-network processing, to data modelling, analysis and applications. It reflects the cross-disciplinary nature of geosensor networks by bringing together ideas from different fields, such as geographic information systems, distributed systems, wireless networks, distributed databases and data mining.