Such land, and the district classification thereof, shall be shown on the map designated as the "Zoning District Map of Petersburg, Virginia," dated and signed by the mayor and the city clerk of Petersburg upon adoption. This zoning district map, and all notations, dimensions, references and symbols shown thereon pertaining to such districts, shall be as much a part of this ordinance as if fully described herein and shall be filed as part of this ordinance by the clerk of the City of Petersburg. Said map shall be available for public inspection in the office of the city clerk, and any later alterations of this map, adopted by amendment as provided in this ordinance, shall be similarly dated, filed and made available for public reference.