Data is received that describes a polyline having a first endpoint, a second endpoint, and a plurality of intermediate vertices, each of the intermediate vertices lying between the first endpoint and the second endpoint. An estimation line segment is drawn between the first endpoint and the second endpoint. An intermediate vertex is identifies as a pivot vertex from the plurality of intermediate vertices that is a greatest distance from the estimation line segment. A flatness ratio is calculated by dividing a distance of the pivot vertex from the estimation line segment by a length of the estimation line segment In a computer, the flatness ratio is compared to a predetermined threshold value. If the flatness ratio does not exceed the predetermined threshold value, the intermediate vertices are discarded, thereby modifying the polyline.