The zone boundaries are either streets or alleys, unless otherwise shown, and where the designation on the maps accompanying and made a part of this chapter indicating the various zones are approximately bounded by street or alley lines, said street or alley shall be construed to be the boundary of such zones.

Where the zone boundaries are not otherwise indicated and where the property has been or may hereafter be divided into blocks and lots, the zone boundaries shall be construed to be lot lines, and where the designations on the maps accompanying and made a part of this chapter indicating the various zones are approximately bounded by lot lines, said lot lines shall be construed to be the boundary of such zones unless said boundaries are otherwise indicated on the maps.

Whenever a portion of any zone is indicated upon the zoning map as a strip paralleling an opened or unopened street, the width of this strip, unless delimited on said map by lot lines or otherwise, shall be assumed to be 120 feet measured at right angles from the lines of the street to which it is parallel and adjacent.