The datum plane of all grade elevations in the city is hereby declared to be on the same level as the top of a certain anchor iron now imbedded in the coping stone on the north side of the lower lock of the Illinois and Michigan Canal, known as lock number 15, said anchor iron being the north 1 of 2 anchor irons imbedded in the coping immediately east of the west gate of the lock; and the datum plane shall be known as 0 for all elevations.
(1963 Code, § 8-2-4)