There are 10 raingauge stations available to calculate the rainfall characteristics of a catchment whose shape can be approximately described by straight lines joining the following coordinates (distances in kilometres): (30, 0), (80, 10), (110, 30), (140, 90), (130, 115), (40, 110), (15, 60). Coordinates of the raingauge stations and the annual rainfall recorded in them in the year 1981 are given below. ENGGA Station Co-ordinates Annual Rainfall (cm) Station Co-ordinates Annual Rainfall (cm) Determine the average annual rainfall over the catchment. (0,40) 132 6 (0.80) 124 2 (50, 0) 136 7 (40, 50) 3 (140, 30) 156 93 8 (90,30) 128 (140, 80) 81 9 (90,90) 102 5 (90, 140) 85 10 (40,80) 128