Where the boundary lines are shown approximately on the location of property lines, such property lines shall be the boundary of the district. Where the boundary lines are shown approximately 100 feet from the street lines and it appears that they are not intended to follow the property lines, then the boundary lines shall be regarded as being 100 feet distant from the street lines. In cases not covered by these provisions, the location of boundary lines shall be determined by the distances in feet, if given, from other lines on the map or by the scale of the map if no distances in feet are given.