The boundaries of said districts shall be shown upon the map on file in City Hall which shall be designated "Zoning Map." The said map and all the notations, references and other data shown thereon are hereby incorporated by reference into this chapter as if all were fully described herein.

The boundaries between districts are, unless otherwise indicated, either the center lines of streets, alleys, railroad rights-of-way, streams of 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. Where no figures are shown and no other definite boundaries are indicated, then scaled distances shall be used.

When a district boundary line divides a lot held in single and separate ownership at the effective date of this chapter, the regulations as to the use in a less restricted district shall extend over the portion of the lot in the more restricted district a distance of not more than 50 feet beyond the district boundary line.