The key difference between the Lake and the other two features is that the former forms a closed contour. Assuming that the shapes are processed one at a time, basic two-step approach for differentiating between the three shapes is as follows:

Step 1. Apply an end-point detector to the object until convergence is achieved. If the result is not the empty set, the object is a Lake. Otherwise it is a Bay or a Line.

Step 2. There are numerous ways to differentiate between a lake and a line. One of the simplest is to determine a line joining the two end points of the object. If the AND of the object and this line contains only two points, the figure is a Bay. Otherwise it is a line segment. There are pathological cases in which this test will fail, and additional }intelligence} needs to be built into the process, but these pathological cases become less probable with increasing resolution of the thinned figures.