The external boundaries of a subdivision shall be monumented in the field by survey markers placed not more than 1,400 feet apart in any straight line, at all corners, at each end of all curves, at the point where a curve changes radius, at all angle points along the meander line, those points to be not less than 20 feet back from the bank of any river or stream or edge of wetland, except that when such corners or points fall within an existing street, the survey markers shall be placed in the side line of the right-of-way.