A monument shall be accurately placed at each corner and at changes in direction of the boundary at each street intersection, at points of curves of streets, and at intermediate points and corners of lots and at other locations as may be required by the City Engineer. The monuments shall be concrete, 6" x 6" x 30" long with an iron pin in the center or metal monuments as approved by the City Engineer. The top of the monument shall be set at the finished grade upon the completion of the grading of the streets and the lots.