A Chicago city ordinance that prohibits posting commercial advertisements on lampposts, fire hydrants and other public surfaces is unconstitutionally vague, a federal judge has held.In a written opinion Saturday, U.S. District Judge Matthew F. Kennelly ruled the ordinance runs afoul of the due process clause because it fails to define “commercial advertising material.”Without such a definition, Kennelly wrote, quoting Sherman v. Koch, 623 F.3d 501 (7th Cir. 2010), the ordinance “fails to define the …