The Drainage Analogy

The topographical watershed concept is based on an analogy that all points on the surface are classified according to drainage, i.e.  any two points are in the same region if they drain to the same point.

Courtesy of Dr. Peter Yim at National Institutes of Health, Bethesda, MD

 Left: Gradient image of some object image.  Object boundary can be viewed as an area where pixel intensity changes at the greater rate than object interior so that gradient corresponding to object boundary is higher, whereas gradient values in object interior are lower since the intensity does not change greatly. This gradient image can be viewed as a simple topographical surface if the gradient value is taken as the height.

Center: Downstream.  If a drop of rain falls on a point P, it flows down the surface following a line of steepest descent called downstream and stops at some local surface minima (M).   If a drop of water falls on another point P' and stops at the same M, these two points belong to the same watershed. 

Right: Watershed lines.  Adjacent watersheds are divided by a path where a drop of water has the equal chance of flowing into adjacent watersheds.  The path represents watershed line (W) that usually occurs along the peaks of ridgelike structures.