The cache is not at the posted coordinates as that would require a boat, which is not permitted on this stretch of the Quoile.
To find the correct co-ordinates simply obey the following instructions.
Cut the square in half. Place one piece on top of the other.
Cut the pieces in half at 90 degrees to your first cut. Place two pieces on top of the other two.
Cut again in the same direction as your first cut. Place one pile on top of the other.
Cut again at 90 degrees to your first cut. Place one pile on top of the other.
Taking from the top and working left to right, deal out all the squares.
If you have cut and stacked the pieces correctly, you should be able to read the co-ordinates and add them to Certitude.
You can validate your puzzle solution with certitude.