The Loop District in Chicago in Cook County, Illinois — The American Midwest (Great Lakes)

Number One City Datum

Photographed by Sean Flynn, June 10, 2024

1. Number One City Datum Marker

Inscription.
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 granite base at the southeast corner of the Northern Trust Company Bank Building at the northwest corner of S. LaSalle Street and W. Monroe Street."
September 12, 1928
Municipal Code of Chicago

Topics. This historical marker is listed in these topic lists: Science & Medicine • Settlements & Settlers. A significant historical date for this entry is September 12, 1928.

Location. 41° 52.847′ N, 87° 37.949′ W. Marker is in Chicago, Illinois, in Cook County. It is in The Loop District. It is at the intersection of South LaSalle Street and West Monroe Street, on the left when traveling east on South LaSalle Street. The marker is at the southeast corner of the Northern Trust Building, facing Monroe Street. Touch for map. Marker is at or near this postal address: 50 South LaSalle Street, Chicago IL 60603, United States of America. Touch for directions.

Regionally, this marker is in the

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American Midwest

and on the Great Lakes.

Globally, it is in North America,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Historically, it finds itself in what was once the Viceroyalty of New France,

the territory of the Mississippian Culture,

and the Northwest Territory.

Other nearby markers. At least 8 other markers are within walking distance of this marker: Roanoke Building and Tower (about 300 feet away, measured in a direct line); Field Building (about 400 feet away); One North LaSalle Building (about 600 feet away); Continental and Commercial Bank Building (about 600 feet away); Rookery Building (about 600 feet away); Marquette Building (approx. 0.2 miles away); The Standard Time System in the United States (approx. 0.2 miles away); Burnham Center (approx. 0.2 miles away). Touch for a list and map of all markers in Chicago.

Regarding Number One City Datum. This point at the southeast corner of the Northern Trust building is used as the primary "bench mark" for Chicago's elevation. According to the Chicago Public Library, Chicago's elevation ranges from 577 feet at the surface of the Chicago River

Northern Trust Bank Building: 597.52 feet above sea level image. Click for full size.

Photographed by Sean Flynn, June 10, 2024

2. Northern Trust Bank Building: 597.52 feet above sea level

According to the City of Chicago, this building is 597.52 feet above sea level. The City Datum marker can be seen in the corner of the building, towards the bottom of this photo.

to about 670 feet at a location on the city's far southwest side; this spot at the Northern Trust building is 597.52 feet above sea level. The city datum at Monroe and LaSalle is one of several bench marks that are laid out around the city to monitor the city's elevation; link 2 below lists all of those bench marks, their locations and the elevations.

A 1949 Chicago Tribune interview with a 79-year-old engineer named Charles Jones, who claimed to have originally marked the Northern Trust building, offers some background and history related to the building's selection as the "city datum." Jones told the newspaper that datum was set at the low-water mark of Lake Michigan in Chicago, in 1847: "579.88 feet above the mean tide at New York." All construction in Chicago is governed by this datum to ensure that sewers are set at the right level. "The city is laid out," he said, "so that gravity controls the flow of sewage from private drains into city sewers, thence into the sanitary district's intercepting sewers, and so that storm water runs from the sewers into the river." The Northern Trust building was selected, Jones said, because it

Photographed by Sean Flynn, June 10, 2024

3. Number One City Datum Marker

La Salle Street heads north in the background of this photo.

was built on caissons that go down to solid bedrock, meaning that there is believed to be no likelihood that the building will sag, and the bench mark will remain stationary.

Also see . . .
1. The Elevation of Chicago: A Statistical Mystery. The Chicago Public Library explains how elevation is measured in Chicago and the difficulty in finding an exact measurement for how far above sea level the city is.

Excerpt: "Ground level in Chicago ranges from 577 feet at the surface of the Chicago River to approximately 670 feet near 103rd and Western. Lake Michigan, which is generally a few feet higher than the Chicago River, has recently varied from 576 to 582 feet above sea level. Several residential areas along the south lakefront are close to 580 feet—or lake level.

But people live and work at higher and lower elevations. The area around Willis Tower provides a good illustration. The ground level is about 595 feet. The tower’s antenna is 1730 feet. The floor of the lowest basement is 43 feet below ground level. So the tower ranges from 552 to 2325 feet above sea level. Chicago has 7 buildings over 900 feet tall, giving it a
Chicago Daily News Collection, Chicago History Museum, circa 1906

4. Northern Trust building under construction
The Northern Trust building opened on September 1, 1906, at the corner of La Salle and Monroe streets in Chicago's Loop. It was designed by Frost and Granger, a Chicago architectural firm best known for designing train stations across the Midwest. The building was selected as Chicago's elevation "bench mark" because of how it was constructed: Its caissons go down to solid bedrock, and city engineers believed there was no likelihood the building would sag, which means that the bench mark would remain stationary. According to the City of Chicago, this bench mark is 597.52 feet above sea level: 17.64 feet higher than Lake Michigan's low-water mark in 1847, which was 579.88 feet above the mean tide at New York.
mini mountain range 1500 feet above sea level with higher summits."

(Submitted on June 10, 2024, by Sean P. Flynn of Oak Park, Illinois.)

2. Chicago Elevation Benchmarks. From the City of Chicago's Department of Water Management (Submitted on June 10, 2024, by Sean P. Flynn of Oak Park, Illinois.)

Credits. This page was last revised on July 5, 2025. It was originally submitted on June 10, 2024, by Sean P. Flynn of Oak Park, Illinois. This page has been viewed 2,245 times since then and 271 times this year. Photos: 1, 2, 3. submitted on June 10, 2024, by Sean P. Flynn of Oak Park, Illinois. 4. submitted on August 21, 2024, by Sean P. Flynn of Oak Park, Illinois.