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Odd borders

See also the following link where the distintiction between delimited and demarcated borders is explained:

• delimited borders: properly fixed, the countries on either side have agreed to the exact physical location of the border, and the coordinates of that physical line have been thoroughly and accurately documented

• demarcated borders: physically marked on the landscape, with reinforced concrete pillars, anchored by underground footings.

https://www.atlasobscura.com/articles/cameroon-nigeria-border-dispute

Odd borders

And even today, not even such protocols exist. Many borders are not defined in such a precise way, but are still based on older principles like "in the middle of the river" - riverbeds change, however. There are sections of the border between Switzerland and Italy that are defined as "the ridge of a glacier". With climate change and melting glaciers, the border changes, too. As well, there is no agreed definition of the border between Switzerland and Germany in the Lake of Constance - as there is no need for it:

And even today, not even such protocols exist. Many borders are not defined in such a precise way, but are still based on older principles like "in the middle of the river" - riverbeds change, however. There are sections of the border between Switzerland and Italy that are defined as "the ridge of a glacier". With climate change and melting glaciers, the border changes, too: Melting glacier in Alps shifts border between Switzerland and Italy. 

As well, there is no agreed definition of the border between Switzerland and Germany in the Lake of Constance - as there is no need for it:

Unmarked segments of (uncontested) borders

This article makes it clear that no map is precise enough to establish a border - borders are

established on the basis of international agreements.

Unmarked segments of (uncontested) borders