Except where referenced on the 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 adoption of this chapter, but where a district line obviously does not coincide with the lot lines as such, or center lines of streets or alleys, or where it is not designated by dimensions, it shall be determined by the use of the engineer's scale as measured on the official zoning map.