(a)   The district boundary lines on the District Map are intended to follow either center lines of streets or alleys or lot lines.  Where the districts designated on the Map or bounded approximately by the street, alley, or lot lines, the street, alley, or lot shall be construed to be the boundary of the district, unless the boundary is otherwise indicated on the Map.  In the case of unsubdivided property, the district boundary lines shall be determined by the use of the scale appearing on the Zoning District Map or by dimensions.
   (b)   Where the boundary of a district follows a railroad line, the boundary shall be deemed to be located midway between the main tracks of the railroad line.