[culebron21]

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.

[Someone]

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)