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Do west side streets jog at North Avenue because of the curvature of the earth?
October 8, 2009

Dear Cecil:

You filthy lying pig, you're a fraud and I can prove it. More than 30 years ago, when your output was exclusively daubed in toxic resins on the corpses of butchered forests, you shamelessly retailed the old Chicago legend that major west side thoroughfares such as Austin Boulevard, Central Avenue, etc., jog sharply at North Avenue to account for the curvature of the earth. Modest research would have been sufficient to demonstrate that this tale is in the same league as Mrs. O'Leary's cow and the 90,000 dead in that 1885 cholera epidemic. Try again, chump.

— C. Adams, East Illinois Street

 

Cecil replies:

I'm so ashamed.

You're thinking: of course you are. You're writing letters to yourself in the newspapers. But that's only part of it. The main reason I'm ashamed is that my overbearing conscience is right: more years ago than I care to tell you about, I passed along a charming legend without close examination, thereby condemning the youth of Chicago (also the youth of Baltimore, Washington, and Los Angeles, where the Straight Dope was syndicated at the time — God knows what they made of it) to ignorance-blighted lives. Well, it's time to come clean.  First, however, I'm going to explain the legend in all its lurid detail, because it is, in its way, a beautiful thing. Is it a tall tale? Please. Paul Bunyan is a tall tale. This one involves the curvature of the earth, and they don't come much taller than that.

The setup is this. As you know if you've spent much time on the west side, the major north-south streets do indeed swerve abruptly on crossing North Avenue, 1600 north. That's true not just of Austin and Central mentioned above, but every half-mile commercial street from Kedzie to Oak Park Avenue, a distance of four and a half miles.

The explanation I heard for this in high school was as follows. The Chicago street layout is of course a grid. This grid is overlaid on on a sphere, namely the earth. All grids overlaid on spheres present the difficulty that as the meridians (the north-south lines) approach the poles, they converge. If you're a surveyor laying out a street grid you have two choices. Either you let the streets get closer together as they head poleward, meaning the building lots get smaller and thus can be sold for less, or you readjust the grid at intervals so the lots, and thus the profits, don't shrink. Chicago being Chicago, the obvious choice was #2. The readjustment, or so the story has it, is manifested most conspicuously at North Avenue.

You can see the appeal of this tale for impressionable minds. Here's a city so vast and expansive — and also, frankly, so flat — that in simply deciding where the streets go one must take account of the geometry of the planet. So delighted was I with this apparent state of affairs that when, as a cub omniscient, I received a letter raising this very question, I repeated the explanation just as I'd heard it. On maturer thought, however, I was obliged to concede: I should have scrutinized that one more than I did.

Indeed. It breaks my heart to admit this, but the story is probably wrong.

My source here, as with all matters having to do with Chicago geography, is mapmaker Dennis McClendon. Dennis acknowledges that the full story will never be known with certainty, but contends the more likely explanation for the jogging streets isn't the curvature of the earth, the Coriolis effect, or any other such hifalutin subject. He blames incompetent surveying — and as soon as the words were out of his mouth, I knew he was right.

Let's grant the story has a germ of truth to it. Here we must descend into the rankest map geekery, but by God we're going to get to the bottom of this. The Public Land Survey System, which is used in most of the U.S. and on which the Chicago street grid is largely based, does in fact need to account for the curvature of the earth. But the amount of adjustment is small. Dennis has come up with a precise calculation I'll get to in a minute, but for now let's just say it's a little more than a foot per mile. Survey corrections are generally made every 24 miles, at township boundaries. So the necessary adjustment is roughly 30 feet.

This is much less than the observed offset at North Avenue. At Austin Boulevard, for example … well, I didn't have my surveyor's transit with me, but my guess is that at North Avenue the jog is on the order of 150 feet. Explaining this enormous discrepancy requires a force considerably greater than the curvature of the earth. Stupidity has got to be it.

Granted, the evidence for this is circumstantial. However, we know the following:

  • North Avenue is indeed a township boundary. The townships to the north and south of it were surveyed at different times by different people.

  • The guy who laid out the south township appears to have been a bit of a jamoke. Harlem Avenue, the township's western border, is 1/16 mile west of where it should be at Madison Street, says Dennis.  That's 330 feet. Just guessing, but if I had to hide a 330-foot screwup, I'd parcel it out in dribs and drabs along the township's six-mile width. Does that give us a 150-foot offset at Austin Boulevard? I'd say it gets us pretty close.

  • It's not like this is the only surveyor's error in Chicago.  The whole damn town is 1.3 degrees off true north. As a result, it doesn't square with the survey grid between the Wisconsin line and Central Street in Evanston, which was laid out independently. Evidence: Central and Golf Road are supposed to be parallel. However, if you follow the lines in your Rand McNally street atlas — everyone should have a Rand McNally street atlas — you find Central and Golf are 1.5 miles apart in Elgin but only a half mile apart in Evanston.

Let's not be too hard on these guys. At the time, who cared about the difference? They thought they were surveying farm property lines, not crosstown arterials for some sort of trackless personal locomotive or other deviltry that the distant future —  like the 20th century — might bring.

So there you go. The burden of guilt is lifted from my mind. This has necessitated my kiboshing a cherished story from my youth, but in my opinion I've replaced it with a better one. As bonus, we'll now add to the world's store of scientific precision, or at least Dennis will, thereby demonstrating he's one of the great minds of our time. It's all very well for a newspaper columnist to speak of survey adjustments of "a little more than a foot per mile." That won't do if you're a cartographer. Summoning occult resources (actually, Ed Williams's Great Circle Calculator at williams.best.vwh.net/gccalc.htm), Dennis informs me that the survey correction required every 24 miles to account for the curvature of the earth at our latitude is (get ready):

28.4669115 feet

(theoretically). I advise you to write this down. If we ever need to lay out this town again, we'll want to get it right.  

— Cecil Adams
Photo by Pat O'Neil

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