All heights within Chicago are measured in relation to the "Number One City Datum", a mark on the wall of the Northern Trust Co, at LaSalle and Monroe

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Surveyors use this point, a mark cut onto the granite wall, which is just over 3' above ground, as the primary reference point, or "datum", when determining heights throughout the city.

The Northern Trust Co building was chosen as the number one city datum because it is built on bedrock, and thus more stable.