This paper presents a system for monitoring quasi-static displacements, named GWSN for "GPS wireless sensor network". This system consists of the technologies of a low-cost L1 GPS (Global Positioning System) and wireless sensor network. The low-cost sensor node and cable-less system enable us to deploy sensors in dense numbers over a vast area. In general, displacement monitoring using GPS has a trade-off relationship between energy consumption and accuracy of estimation. Measuring continuous data and applying a trend analysis or Kalman filter outputs a highly accurate displacement estimation but also needs much energy dissipation. To solve the trade-off relationship, a data compression method and a static GPS positioning analysis method were developed and implemented in the system. In these methods, the assumption of quasi-static displacement was taken into account. A prototype system was developed and experiments were conducted in a quarry site and in an ideal condition using the developed prototype. The experimental results show that the prototype system works without serious system trouble in outdoor environments and has a great potential to measure displacements with sub-centimeter accuracy.