The boundaries between districts are, unless otherwise indicated, either the center lines of existing cartways, alleys, railroad rights-of-way, stream channels or such lines extended, or lines parallel thereto as shown on the Zoning Map. Where figures are shown on the Zoning Map between a street and a district boundary line, they indicate that the district boundary line runs parallel to the street line at a distance therefrom equivalent to the number of feet so indicated. Where no figures are shown and no other definite boundaries are indicated on the Zoning Map then scaled distances shall be used in accordance with the scale shown on the Zoning Map.