It was a geological map of Germany, and with some trouble I found out where I was.

I was an enormous distance from my goal and moreover I was clean off the road to the East.

To go there I must first go to Bavaria and then into Austria.

I noticed the Danube flowing eastwards and remembered that that was one way to Constantinople.
Then I tried another map.

This one covered a big area, all Europe from the Rhine and as far east as Persia.