UnityEditor.HeightmapFilters.Smooth C# (CSharp) Method

        public static void Smooth(TerrainData terrain)
        {
            int heightmapWidth = terrain.heightmapWidth;
            int heightmapHeight = terrain.heightmapHeight;
            float[,] heights = terrain.GetHeights(0, 0, heightmapWidth, heightmapHeight);
            Smooth(heights, terrain);
            terrain.SetHeights(0, 0, heights);
        }