The boundaries between districts are, unless otherwise indicated, either the centerlines of streets or railroad rights-of-way, or such lines extended, or lines parallel or perpendicular thereto. Where figures are shown on the zoning map between a street line and a district boundary, they shall indicate that the district boundary runs parallel to the center-line of the right-of-way of the street at a distance therefrom equivalent to the number of feet so indicated. Where the boundaries of a single district are indicated as including directly opposite sides of a street, lake or watercourse, or right-of-way of a utility line, for any portion of its length, the district so indicated shall be construed to apply to the entire bed of such street, lake or watercourse, or right-of-way of such utility line, lying within such portion of its length. Where uncertainty exists as to the location of any boundaries shown on the zoning map, the following rules shall apply: