Fig. 1
From: Geographical model-derived grid-based directional routing for massively dense WSNs
Grid point network of ROI. The ROI is divided into equally spaced grids which compose a grid point network (grid points are connected by dashed lines). 
is the grid point located at the ith column and the jth row. A node is defined as belonging to 
if its closest grid point in the ROI is 
; for example, all nodes in the dark gray region belong to 
. The path indicated by the blue solid line is the minimum cost routing path from 
to the grid point which the sink belongs to (indicated by the red circle). The black region represents the hole (the region without enough working sensors)