Number One City Datum Point, Chicago

I grew up in the Chicago suburbs and spent much time loose in the city as a kid, and never heard of the “Number One City Datum”, which is in the Northern Trust building at LaSalle and Monroe. It defines a point about 17 feet below street level as the basis point for all height determinations in Chicago (given that “street level” is itself a vague term in the loop, being below street level isn’t surprising). The other surprising thing is that this isn't the 0,0 point in the Chicago grid (which is State and Madison).

More about the Number One City Datum point: www.ntrs.com/aboutus/history/nthist1889.html