In later years, as punch card data was converted to magnetic tape files the sequence numbers were often used as a column in an array as an index value that can be correlated to time sequences, such as in the natural sciences where the data on the cards were related to the periodic output of a measuring device such as water stage level recorders for rivers and streams in hydrology, or temperatures in meteorology. Entire vaults full of card decks could be reduced to much smaller racks of nine-track tapes.