Monuments shall be placed at all street intersections, boundary angle points of curves in streets, and at such intermediate points as required by the City Engineer. The monuments shall be of concrete filled pipe or tile, weighing at least fifty (50) pounds, capped with standard markers. Street monuments shall be set between six inches (6") and one foot (1') below official furnished street grades and in paved streets shall be enclosed in a standard monument case. If a monument is placed in an open field or unpaved street, the land surveyor shall place an iron pin in the center of the concrete or tile pipe.