Monuments shall be placed at all corners and angle points of the outside boundary but no further than one-quarter (1/4) mile apart. The monuments shall be of concrete, not less than six inches (6") in diameter and thirty six inches (36") deep, with a center copper dowel three inches (3") long cast in place. Iron pipe or steel bars not less than three-quarter inch (3/4") in diameter and thirty six inches (36") long shall be set at the intersection of street centerlines and at all corners of lots not marked by monuments.