The boundaries between districts are, unless otherwise indicated, either the center lines of streets, lanes, watercourses, property lines and rights-of-way of power lines, railroads and other public utilities or of such lines extended or lines parallel thereto. Where the boundaries of a single district are indicated as including directly opposite sides of a street, lane, lake, watercourse, or right-of-way of a power, line or other public utility or any portion of its length, the district indicated, shall be construed to apply to the entire bed of such street, lane, lake or watercourse, or right-of-way of power line, railroad or other public utility lying within a portion of its length.