(A) Bench mark established. The datum plane of all grade elevations in and for the city shall be and the same is hereby declared to be a plane on the same level as the top of the southwest corner of the south concrete abutment of the bridge over the Illinois & Michigan Canal on Main Street in the city, which is hereby declared to be the bench mark for said purpose and said datum plane shall be known as 66.373 for all elevations.
(B) Grade for Main Street. Commencing at a point 15 feet north of the south line of Bluff Street in the city which is 17.627 feet above the datum plane heretofore established in division (A); thence south along the centerline of Main Street to the intersection of said Main Street with the south line of Clark Street, at a grade of 2.8%; thence south along the centerline of Main Street on the south line of Washington Street at a grade of 3.1%; thence south along the centerline of Main Street to the north track of the Chicago, Rock Island and Pacific Railroad at a grade of 3.7%; thence south along the centerline of Main Street to the south end of the bridge over the Illinois & Michigan Canal at a grade of 0.25%; thence south along the centerline of Main Street to the north end of the north race bridge at a grade of 1.8%; thence south along the centerline of Main Street to the north line of River Street, at a grade of 0.6%; thence south along the centerline of Main Street to the north end of the bridge over the Illinois River at a grade of 4%.
(C) Other grades. All other grades established in any of the streets of the city shall conform with and be established from the datum plane herein established.
('72 Code, § 6.03)