The water divides above 3500 m traversing perpetual snowfields and glaciers have shifted during the past decades, partly due to melting glaciers. The following aerial photograph and topographic map show the boundary between Switzerland and Italy at the Furggsattel in Zermatt. The crosses depict the boundary in 1940 and the new course is shown in red.
The graphic representation illustrates how the courses of boundaries around Zermatt are influenced by melting glaciers. From 1940 to 2000, the height of the glacier has decreased and the water divide now follows the rocks. This decrease causes a shift in the boundary of about 100 to 150 m.
Only approx. 40 km of the water divides between Switzerland and Italy (total length 578 km) traverse snow fields or glaciers. There are no snow fields or glaciers along the rest of the alpine boundaries between France and between Austria which might result in any modifications. The entire boundary passes across solid terrain.
From the point of view of the geometer, these boundary modifications are due to natural phenomena which must be taken into account in order to guarantee the correct revision and update of our topographic and cartographic products. However, it is not possible to make new surveys every few years; therefore, such updates are only carried out as the need arises.