def __init__(self, data_folder: str, prj: 'hyo2.soundspeed.soundspeed import SoundSpeedLibrary') -> None:
        self.name = self.__class__.__name__
        self.desc = "Abstract atlas"  # a human-readable description
        self.data_folder = data_folder
        self.prj = prj
        self.g = Geodesy()

        self._has_data_loaded = False  # grids are "loaded" ? (netCDF files are opened)

        self._file = None
        self._day_idx = 0
        self._timestamp = None
        self._zeta = None
        self._siglay = None
        self._h = None
        self._lats = None
        self._lons = None
        self._lat = None
        self._lon = None
        self._loc_idx = None
        self._d = None
        self._temp = None
        self._sal = None