Going on adding a few smileys on the map, with riddles I could finally solve. Although this is the kind of riddles I should like, I put this one aside for a while, as I somehow didn't find the right way to start. I did have a good starting point, but somehow decided to use the wrong values at first. As I finally considered more options, I could eventually make enough headway to get the coordinates.
At ground zero (well, not a fully unknown area now ), the task was very easy and I could soon sign the logbook.