Hole Detection and Shape-Free Representation and Double Landmarks Based   Geographic Routing in Wireless Sensor Networks
  In wireless sensor networks, an important issue of Geographic Routing is local minimum problem, which is caused by hole that blocks the greedy forwarding process. To avoid the long detour path, recent research focuses on detecting the hole in advance, then the nodes located on the boundary of the hole advertise the hole information to the nodes near the hole. 
Jianjun Yang, Zongming Fei, Ju Shen
cs.NI