This bench mark designates the primary datum for the city of Chicago. It is from this point that all heights in the city are measured.

 
"The City Datum is hereby established as a plane 17.640 feet below the bench mark cut on top of the bottom stone of the granite base at the southeast corner of the Northern Trust Company bank building at the northwest corner of South Lasalle Street and West Monroe Street"

September 12, 1928

Municipal Code of Chicago