Abstract: The relevant edge nearest to the given point is determined by first projecting the point and polygon edges onto X-axis and finding the edges within whose ranges the point projection is located. Then the point and relevant edges found in the first step are projected onto Y-axis to pick out the nearest edge PiP_i+1 from the given point Q.If the orientation of triangle PiP_i+1 Q is the same as polygon, Q is inside the polygon.Otherwise,it is outside the polygon.Compared to Jordan curve theorem algorithm,this approach is more efficient and reliable,especially in the case of large number of polygon edges.