public void SetGroundTile(GameObject sprite, int x, int y)
{
Tile tile = this.tileMap[x, y];
Destroy (tile.ground);
tile.ground = Instantiate (sprite,
new Vector3(x - this.columns / 2 + .5f, y - this.rows / 2 + .5f, 0f),
Quaternion.identity) as GameObject;
tile.ground.transform.Translate(new Vector3(0, 0, y));
}