Friday, April 24, 2020

Why the North-South Chicago Streets Don't Align at North Avenue.

Chicagoans know that the north-south streets swerve abruptly on crossing North Avenue (1600N) from Kedzie Avenue (3200W) on the east to Oak Park Avenue (6800W) to the west, a distance of 4½ miles.
Chicago's streets are a grid pattern that is then overlaid on a spherical map of the Earth. All grids overlaid on spheres create an issue. They converge as the meridian's lines (north/south) approach the poles.

A surveyor laying out a grid has two choices. Either let the streets get closer together as they head towards the North Pole, meaning the building lots get smaller and thus would be sold for less money, or readjust the grid at intervals so that the lots and the profits don't shrink.

Chicago opted for the second choice. The abrupt readjustment, or so the story has it, is manifested most conspicuously at North Boulevard, which was at one time the northern boundary for the City of Chicago.
Looking North on Pulaski Road (Crawford Avenue) towards North Avenue, Chicago - 1947
Different people surveyed the townships to the north and south of North Avenue at different times.

The surveyors who laid out the city south of North Avenue were inaccurate. Harlem Avenue, the city's western border at that time, is 1/16 miles west of where it should be at Madison Street. That's 330 feet. If one had to hide a 330-foot mistake, they might parcel it out in small increments along the 5-mile width.

It's not like this is the only surveyor's error in Chicago. The whole city is 1.3° off true north. As a result, it doesn't square with the survey grid between the Wisconsin border and Central Street in Evanston, which was laid out independently.

Central and Golf Road are supposed to be parallel. However, if you follow the lines in a Street map or atlas, you'll find Central and Golf are 1½ miles apart in Hoffman Estates but only ½ mile apart in Evanston.

Who cared about such a slight difference at the time of the survey? They thought they were surveying farm property lines. They couldn't have imagined what the future would bring.

U.S. surveyors in 1800 did not know that the magnetic North Pole moves constantly. While there were observations of compass deviations from true north throughout history, the concept of a continuously moving magnetic pole wasn't widely understood or accepted until the late 19th century.

English seafarers observed discrepancies between geographic and magnetic north in the 16th century.

Edmund Halley observed that the magnetic pole wasn't fixed and proposed its movement over time in the 17th century.

By the 18th century, scientists had observed that the compass needle didn't always point directly to the true north. This deviation, known as magnetic declination, varied depending on location and was attributed to local magnetic anomalies.

Scientific data on the magnetic North Pole's movement was still limited and largely based on single expeditions by 1800. Many surveyors and scientists believed the magnetic pole had a fixed location or followed a circular path. Surveyors in the 1800s primarily relied on compasses and declination tables to adjust for local variations without necessarily factoring in long-term movement.

U.S. surveyors in 1800 observed discrepancies between magnetic and geographic north but lacked a complete understanding of the cause. The concept of the magnetic North Pole's movement was still emerging and not widely accepted during their time.

Increased data from expeditions and more precise instruments started to solidify the evidence of the magnetic pole's continuous movement in the Mid-19th century.

The concepts of "secular variation" (long-term movement) and "diurnal variation" (short-term changes) gained wider acceptance in the late 19th century.

Carl Friedrich Gauss (1777-1855), the renowned mathematician, made significant contributions to our understanding of Earth's magnetic field, particularly regarding the location of magnetic north. In the 1830s, Gauss participated in the first global survey of Earth's magnetic field. This required accurate measurements, which led him to invent the magnetometer, a device to measure the direction and strength of magnetic fields. Gauss recognized that the location of the magnetic north is not fixed but drifts over time. Gauss's work on Earth's magnetism extended beyond just the north pole. He also studied magnetic declination, the angle between true north and magnetic north, and its variations across the globe.

ADDITIONAL READING: Chicago's Eastside Begins at North Boulevard (1600N)

Compiled by Dr. Neil Gale, Ph.D.


  1. I read a fascinating book recently “Measuring America” by Linklater and find the whole process of metes and bounds so interesting. This is a great article. Surveying was a job done by good or sloppy workers, honest or dishonest workers.

  2. A good article, Neil. Townships and township sections are sometimes offset as well for similar reasons.

  3. Did surveyors 200 years ago know that the magnetic North Pole moves constantly?

    1. Added a section titled; "The History of True North vs. Magnetic North."


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