"Normal peak for Sweden"

Sweden's Normal High Point was a reference point that all other high points in Sweden referred to and was used to be able to make correct indications in terms of height. In the middle of the 19th century, a scientific desire arose to be able to coordinate readings of water level gauges along the Swedish coasts. There was also a great military strategic interest in being able to measure the relative height difference between points in the landscape.

In 1898, the Normal peak was therefore established by Professor P.G. The rose on Riddarholmen in Stockholm. The fact that it ended up there was mostly for reasons of convenience, since the general staff responsible for the project was located at Riddarholmen at the time. Behind the cast-iron hatch on the rock face in Schering Rosenhanes Gränd, there is still today the silver scale that indicates the normal high point of 11.8 meters above sea level.

Today, the zero point is defined by a common height system in Western Europe and the gap in the alley on Riddarholmen is a forgotten relic that few notice. Well worth a stop if you happen to be walking past.
//https://uddautflykter.se/guide/stockholm/normalhojdpunkten/