Monuments shall be placed at all corners and angle points of the external boundaries of a subdivision, but no further than one-fourth mile apart. Unless otherwise specified by the Engineer, the monuments shall be iron rods or pipes at least 30 inches long and two inches in diameter. Solid round or square iron bars of equal or greater length may be used in lieu of pipes wherever pipes are specified in this section. These monuments shall be placed not more than one-fourth mile apart in any straight line and at all lot corners, at each end of all curves, at the point where a curve changes its radius and at all angle points in any line where such corners or points are 100 feet or more apart.