The coordinates will take you to the startingpoint, but in order to take you further you need the "x" and "y" from Fatönk-cache.
Since Fatönk GC4PWVM has been archived you can find the final coordinates for Harpholmen by reading logg of the archived cache!!!
Walk x meters in the direction y°. Now turn 48° towards north and continue 72 meters. You should now be very close to the treasure but it still requires a small effort from you. Be careful!
For easier hunt follow the trail that starts at PieHis, see additional waypoint.