datum vs observance

datum

noun
  • A fact known from direct observation. 

  • A premise from which conclusions are drawn. 

  • A floating reference point, or SLDMB, used to evaluate surface currents in a body of water. Often employed by coastal search and rescue. 

  • A fixed reference point or set of reference points which precisely define a system of measurement or a coordinate system. 

verb
  • To provide missing data points by using a mathematical model to extrapolate values that are outside the range of a measuring device. 

observance

noun
  • That which is to be observed. 

  • The practice of complying with a law, custom, command or rule. 

  • Observation or the act of watching. 

  • A rule governing a religious order, especially in the Roman Catholic church. 

  • The custom of celebrating a holiday or similar occasion. 

  • Reverence; homage. 

How often have the words datum and observance occurred in a corpus of books? (source: Google Ngram Viewer )