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Comment by dantiberian | Hacker News Reader

This is all based on the authors' estimations, not actual measurements or observations.

The scholars first estimated the cumulative weight of New York’s buildings to be 1.68 trillion pounds, and then calculated the downward pressure these buildings exert on the mixture of clay, sand, and slit that make up most of the ground beneath the city’s streets.

Based on their model, New York experiences a “subsidence rate” (the technical term for sinking) of about one to two millimeters per year on average, though Lower Manhattan, as well as particular areas of Brooklyn and Queens, show a propensity for greater subsidence risk.

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Really interesting, How do you do these estimates other than using mathematical models? What kind of measurements do you expect to see?

Land registries have geodesic fixtures in many cities, so geodesists could use them as reference points for construction sites. In German cities, those marks are seen on kerb stones everywhere in the old city centers. They look like a big jeans button (3-4 cm/1" in diameter) with a hole in the center (to let geodesists aim at their centers).

Over couple of decades, such marker would have moved by several cm, relative to such markers in other areas. It would probably be impractical to measure several cm shift over 1-2 km across the river, but fixtures would drift relative to each other, and if there's a trend, it's observable -- geodesists routinely measure the drift between big fixtures in the fields over much bigger distances, and publish updates to their reference coordinates.

See https://en.wikipedia.org/wiki/Survey_marker or (German) https://de.wikipedia.org/wiki/Vermessungspunkt.

They also often are attached to buildings. Partly that’s because high points in the landscape such as church towers are often used for long-distance triangulation, partly because engineers want to track how fast a building sinks relative to its surroundings (such sinkings can be highly local in areas where buildings are built on soft ground or on below-ground sand layers. Bedrock is too far below ground in quite a few locations to build on)

GNSS with ground station assist is capable of measuring changes in elevation within millimeters.

As an example, the US maintains the National Spacial Reference System (NSRS), which is a "consistent coordinate system that defines latitude, longitude, height, scale, gravity, and orientation throughout the United States" with very high accuracy.

https://oceanservice.noaa.gov/facts/nsrs.html

https://earthobservatory.nasa.gov/images/147436/taking-a-mea...

https://pubs.usgs.gov/tm/11d1/tm11-D1.pdf

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