Then the plane right-angle coordinates showed as figure 1 along latitude and longitude direction is built. After a new point received, the MCU calculate the long and latitude of the point, and then use formula 1 and 2 to calculate the coordinate of the point relative to origin. Calculate area and make algebraic sum accumulation by substitution of the coordinate into formula 3 . Then the area of the farmland can be obtained by formula 3 after receiver received all plots of the farmland range.