As the title indicates, I'm confused...

"A traverse le Karst" gives the coordinates of the entrances and exits for the routes it describes, based on "les coordonn?es Lambert". So, I set the coordinate display of my digital copy of the 1:25,000 IGN map of Chartreuse to "Lambert Zone 2", and hover my cursor over the entrance to the Grotte du Mort Ru. The coordinates displayed are: x:878864.31, y:49704.01.

The first 5 digits of the x coordinates compares favourable with the x coordinate in the book, which it gives as 878.87. However, the book's coordinate for the northing is given as 350.31, which appears to have no relationship with the figure displayed on the map.

Can anyone please explain?

My mistake - the northing on the map is actually 50543, so prefixing a 3 to that corresponds reasonably well with that provided by the book.