http://img838.imageshack.us/img838/1538/physicsforumquestion.png [Broken] Say "A" is a river. It is a linear approximation because only several coordinate points are known. The total length is "l" (obtained from the coordinates), "a" represents the distance from the river at which the soil is fertile. The goal is to obtain the total fertile area. If [itex]Area=l*a[/itex] the result is very inaccurate for small angles between lines, as an extra, overlapping, area (represented as red in "B") is added. How could I obtain a more accurate value?