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

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