Hi Evangelos,
If an outline file is given then points outside the polygon in the outline file are set to land.
If an island file is given then points inside the polygon defined by your island file are set to land.
If you just use the bathymetry file, you should get a full matrix of bathymetry
If you add the outline file then land cells are set at the top-right and bottom-left of the matrix
If you add the island file there is a land island in the middle.
The values in the outline and island files don’t get used for the gridding.
The steps are the XYZ file is gridded then any cells that are outside the polygon defined by outline.xyz are set to land and then any cells inside the island file polygon are set to land.
I think you want to add the points from your outline and island files to the XYZ file with a z value of 0.
Chris