Regarding topo map in Basecamp: Have you tried to hover the cursor over the contour lines? Depending on the equidistance the altitude of the contour line should appear.
On my Danish topo map (2.5 m equidistance) the contour altitude will show for every 5 m, i.e. every 2nd contour line. On my South Sweden map (5 m equidistance) the contour altitude shows for every 25 m, i.e. on every 5th line. It is probably similar for your French topo map.
