Except where referenced on the Zoning Map to a street or alley line or other designated line by dimensions shown on the Map, the district boundary lines follow lot lines or the center lines of streets or alleys as they existed at the time of the passage of this Ordinance, but where a district line obviously does not coincide with the lot lines or such center lines or where it is not designated by dimensions, it shall be deemed to be 120 feet back from the nearest street line parallel to which it is drawn.