The shape of a catchment is in the form of a pentagon ABCDE. There are 4 raingauge stations P,Q,R and S inside the catchment. The position coordinates in km are A(0,0), B(50,75), C(100,75), D(150,0), E(75,-50), P(50,25), Q(100,25), R(100,-25)and S(50,-25).If the rainfalls recorded at P,Q,R and S are 88,102,112 and 116mm respectively. Determine the average depth of rainfall over the catchment using Thiessen polygon method.