def bottom(x):         # Triangular island bottom
        H = 10.0
        island_height = 20.0
        island_half_width = 222
        xc = (x[0] + x[-1]) / 2
        distance = np.abs(x - xc)
        b = np.zeros_like(x)
        mask = distance <= island_half_width
        b[mask] = island_height * ( 1 - distance[mask] / island_half_width )
        return - (H - b)