parameters: (self, qtpath, packet_type, channel, layer_id)
docstring: Returns packet at given address and channel.
  
      If packet is not found, throws an exception.
  
      Args:
        qtpath: the quadtree node of the map packet.
        packet_type: the type of data in the map packet.
        channel: the channel of the map packet.
        layer_id: id of layer in the composite globe.
      Returns:
        The map packet itself.
      Raises:
        PortableException: if unknown or unreadable glx.
        UnableToFindException: if unable to find the map data packet.
docstring_summary: Returns packet at given address and channel.
docstring_tokens: Returns packet at given address and channel .
function: def ReadMapDataPacket(self, qtpath, packet_type, channel, layer_id):
      """Returns packet at given address and channel.
  
      If packet is not found, throws an exception.
  
      Args:
        qtpath: the quadtree node of the map packet.
        packet_type: the type of data in the map packet.
        channel: the channel of the map packet.
        layer_id: id of layer in the composite globe.
      Returns:
        The map packet itself.
      Raises:
        PortableException: if unknown or unreadable glx.
        UnableToFindException: if unable to find the map data packet.
      """
      if not self.unpacker_:
        raise portable_exceptions.PortableException(
            UNKOWN_OR_UNREADABLE_GLX_ERROR_MSG % self.globe_name_)
  
      if self.unpacker_.FindMapDataPacket(
          qtpath, packet_type, channel, layer_id, self.file_loc_):
        return self._GetData()
      else:
        raise portable_exceptions.UnableToFindException("Unable to find packet.")
function_tokens: def ReadMapDataPacket ( self , qtpath , packet_type , channel , layer_id ) : if not self . unpacker_ : raise portable_exceptions . PortableException ( UNKOWN_OR_UNREADABLE_GLX_ERROR_MSG % self . globe_name_ ) if self . unpacker_ . FindMapDataPacket ( qtpath , packet_type , channel , layer_id , self . file_loc_ ) : return self . _GetData ( ) else : raise portable_exceptions . UnableToFindException ( "Unable to find packet." )