Each lot and block corner shall be marked with iron pipes or pins not less than one-half inch (1/2") in diameter and not less than eighteen inches (18") long set not less than one inch (1") below the finished grade. Each subdivision corner shall be marked with a permanent concrete marker capped with a noncorrosive metal plate set not less than one inch (1") below the finished grade. (Res. 901, 5-10-1977)