The Swiss will also be adjusting their border with France. The triggers are the rewilding of rivers and a cross-border tram line. Because most international borders were drawn at a time when technical tools were not yet very advanced, many of them are imprecise. A prime example is the border between Canada and the United States along the 49th parallel, known as the "longest straight line in the world" at 2,028 kilometres. Demarcated in the 1870s, the border deviates by up to 175 metres to the north. To the south, the deviation is sometimes as much as 239 metres. In reality, it is the longest zigzag line in the world.

Drawing borders is not only politically but also geographically difficult and sensitive. Alongside (newly emerging) islands, glaciers and snowdrifts in particular repeatedly cause problems, as the mountain ranges beneath them cannot be properly mapped. Without a ridgeline — the line connecting the highest points of a region — and without a watershed, it is often difficult to determine who owns a piece of land. Simply put, when it comes to watersheds, what matters is which way the water flows down the mountain. But glaciers have the power to redirect water. Chile and Argentina, for example, share the third-longest border in the world. Around one per cent, or about 50 kilometres, of it remains undefined, as it lies hidden beneath the not-so-eternal ice of the Campo de Hielo Sur. Climate change will one day resolve the occasional disputes there. 

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