Where district boundaries are indicated as approximately following the center lines or right-of-way lines of streets, highways, public utility easements, and aqueducts or watercourses, said boundaries shall be construed to be coincident with such lines. Such boundaries shall be deemed to be automatically moved if a center line or right-of-way line of such street, highway, public utility, aqueduct, or watercourse is moved a distance of 50 feet or less.