private function _saveBBox($file)
    {
        $this->_saveBBoxRecord($file, 'xmin');
        $this->_saveBBoxRecord($file, 'ymin');
        $this->_saveBBoxRecord($file, 'xmax');
        $this->_saveBBoxRecord($file, 'ymax');
        $this->_saveBBoxRecord($file, 'zmin');
        $this->_saveBBoxRecord($file, 'zmax');
        $this->_saveBBoxRecord($file, 'mmin');
        $this->_saveBBoxRecord($file, 'mmax');
    }