The datum mentioned herein, from which all levels in the village shall be taken, shall be the village datum. Said village datum, for the purpose of establishing grades and elevations, is fixed and hereby established, as a level plane, forty five and one-half feet (45.5') below the bench mark on top of plane monument at the northeast corner of Circle Avenue and Randolph Street, in the village of Forest Park, Cook County, Illinois. No person shall alter the grade of any street, sidewalk or public way. (Village Code § 2-17)