On a topographic map of Chamonix I noticed these thick long-dashed (and sometimes dotted) blue lines. They appear to connect huts, settlements, and some arbitrary points. They tend to follow glaciers.
What are these? ski tours?
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On my Cartes IGN map of the Chamonix area, the map key shows the blue lines as "Cross-country or high mountain skiing route". The footpath lines are shown as solid red lines or dashed red lines when "out of footpath" so presumably the dashed line indicate a footpath route which is not visible on the ground. To me this would imply sold blue lines are "Cross-country or high mountain skiing route" which can be seen on the ground and dashed blue lines show the route taken by the "Cross-country or high mountain skiing route" but it cannot be seen on the ground.