Wireless Sensor Networks are seeing increasing usage in several applications such as military, rescue and surveillance scenarios. Typical for such scenarios is that mobile nodes cooperate side-by-side with stationary sensor nodes to monitor the area of interest and to support the core network operations such as data transport. Global maps of the sensor field, such as temperature and residual energy maps, are of high interest for both users and network designers. However, the map construction can become very inefficient if it requires an extensive intervention of the resource-limited sensor nodes. In this work, we present gMAP, an extremely efficient mobility-assisted approach to construct global maps. In gMAP (a) sensor nodes do not need to process readings of other nodes and (b) require to communicate a minimal number of messages compared to all existing approaches. This is achieved by opportunistically exploiting node mobility to collect data of interest, keeping sensor nodes transmit only their own readings on-demand to a mobile node in their transmission area. ©2009 IEEE.