"Where are we now?"

"Where are we now?"

... and how we find out.

Some thinkers argue that architecture could  be purely speculative and philosophical, not applied, that architecture is wholly distinct from construction, and so architects should deal with the ideas of buildings. There is a tension between the factual and the ideal, and some argue that truth and essence is aligned with the ideal. (I wouldn't so argue, but that's not an issue for this blog.) Even if architecture doesn't need to be in the physical world,  buildings certainly do. Buildings need a where.

Architects don't sweat the exact location of their buildings, but owners must, so engineers and surveyors do. How the where of a structure is determined may vary from place to place, but the observer of architecture should understand how those issues are addressed. How is it done in Chicago?

Some history.

After its 1776 Revolution, the new country of the United States had a huge parcel of land out west:  essentially, the land north of the Ohio River and east of the Mississipppi. The new country also had a huge pile of debts from that Revolution which it could cover by selling this land. But to sell it, the land needed to be converted from just an undifferentiated expanse to a bunch of particular places. 

East, west, north, south.

To do that, the government decided to survey that land by establishing a system of Principle Meridians and Baselines. Meridians were defined north-south lines, and baselines ran east-west. Parcels of land would be defined as having parallel sides to the relevant meridian or baseline. The First Principle Meridian defines the border between Ohio and Indiana, and the Third Principle Meridian was defined as starting at the junction of the Ohio and Mississippi Rivers and runs due north to the Wisconsin border. Most land in Illinois, and all of Chicago, is defined by this meridian. I believe this is why most Chicago streets running north and south actually run a bit more than 1 degree west: earth is approximately spherical, not a plane, so true north along the Third Principle Meridian -- which runs close Normal, Illinois -- is not parallel to true north in Chicago. The divergence is more than one degree and can be detected in Google Maps. 

Why did Chicago inherit its cardinal directions from someplace else? The Third Principle Meridian was defined in 1805, prior to Illinois being a territory in 1809, and well before Chicago was even a recognized place. Back then, everyone was expecting the important places in Illinois to be in the southern parts of Illinois. (Beardstown, Illinois gets its own meridian, the Fourth Principle. The juicy story about Beardstown is that Abraham Lincoln may have contracted syphilis from a prostitute there.)

The baseline for most of Illinois is defined by a line running east-west near Normal, Illinois. 

So that's the origin of the definition of land parcels, and the street grid, in Chicago. We got our cardinal diections from downstate back in the early 1800s.

Up and down.

Chicago is very flat, so up and down are  important only for small differences.  (Perhaps Chicago's skyscrapers were built to fill the psychological gap caused by the absence of mountains:  I will lift up my eyes to the hills, from whence comes my help, as the Psalmist says.)

Water flows downhill, even for very slight slopes, so if water is in any way important to what's going on in the structure, you need to relate to the datum. A datum is an imaginary horizontal plane defined at a particular place. 

The big ones around here are the City Datum, the North American Vertical Datum (NAVD) and the International Great Lakes Datum, 1985 (IGLD85). The Chicago City Datum is a horizontal plane 17.640 feet below a particular benchmark of Lasalle Street. NAVD is defined relative to mean sea level in New York harbour. IGLD85 is defined relative to a bemchmark along the St Lawrence river as it flows into the North Atlantic.

There are many other Datums. (Does the multiplicity of Datums lead to confusion? At the end of the twentieth century, a tunnel was being dug between two buildings in downtown Chicago. But the floors of each half of the tunnel didn't meet: there was about a foot between them. After some finger pointing and public embarrassment, a patch was constructed. I suspect that each building's contractor was using a different datum.)

The NAVD is used in the production of many maps, the IGLD85 is used for planing Great Lakes water flow use, and the City Datum is used for city services. Flood insurance premiums are based on yet other datums.) The IGLD85 and NAVD don't agree exactly: the NAVD is 0.5 feet higher than the IGLD85.  The NAVD will soon be replaced by a system based on the satellites of the Global Position System. This could lead to improvements, certainly, but will not be the end of the story since the ground beneath our feet only looks solid. 

Over the long term, the solid earth is quite elastic. The land around the Great Lakes is rebounding from the latest glaciation. This rebound -- the preferred term is "post-glacial isostatic adjustment"-- allows the bedrock in Chicago to sink, as the liquid rocks deep inside the earth flow north, from which they were squeezed by the now departed glaciers. The land around Chicago has sunk 4 inches in the last century.

Folks who wander through Chicago's Loop may know something about the Chicago City Datum, if they have seen this plaque on the Northern Trust building on LaSalle Street. 

This benchmark is one of hundreds that Chicago maintains. It defines the City Datum as a plane 17.64 feet below the benchmark on the granite.  Number 1 is the only one in the Loop, and likely the only one people have seen. But there is likely another one near you: nearest me is number 565, which (according to its official definition) is "57.5 FEET SOUTH OF THE NORTH LINE OF LEXINGTON STREET" and "30 FEET EAST OF THE WEST LINE OF S LYTLE STREET FROM THE NORTH". 

This benchmark was contructed in 1981, at an elevation of 13.914 feet above the Chicago City Datum, so the elevation around here is about 3 feet lower than the Loop. (The benchmark is actually under the lid.) 

So that's how up and down are determined (at least for now) are determined around Chicago.