On base map only the path and the lake in the dense forest are shown. The ditch from the lake is missing and has to be positioned. The fieldworker measures bearings and distances from the lake to the path abcde but his construction leads to an error of 20 m at the lake. The total measured distance from lake to path is 80 m.
Assuming the error is evenly distributed each point must be corrected in the direction a-a', the correction being in proportion to its distance from the fixed point e to the overall distance.