The boundaries between districts are, unless otherwise indicated, either the center lines of streets or railroad rights-of-way or such lines extended or lines parallel thereto. 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. When the location of a district boundary line is not otherwise determined, it shall be determined by the scale of the map measured from a given line. Where the street layout actually on the ground varies from the street layout as shown on the Building Zone Map, the designation shown on the mapped street shall apply in such a way as to carry out the intent and the purposes of this chapter for the particular area in question.