code: public BaseTile getBaseTile (int tx, int ty)
      {
          BaseTile tile = _base[index(tx, ty)];
          if (tile == null && _defset != null) {
              tile = (BaseTile)_defset.getTile(
                  TileUtil.getTileHash(tx, ty) % _defset.getTileCount());
          }
          return tile;
      }
docs: Returns the base tile at the specified coordinates or null if
  there's no tile at said coordinates.