"datum" and "data"

[James A. Landau]

(this is in reference to a message I can't locate, in which someone described the use of  "datum" in geodesy and cited the plural form "datums".)

At one time "datum"/"data" was a singular-plural pair, but at that time very few people besides the Astronomer Royal had to deal with large collections of data.  Nowadays, that has changed, everybody's work is full of "data", and your home computer has "database software".

My hypothesis is that "datum" and "data" have become detached and are now separate words.

"data", like "information", is a mass noun and has no singular or plural form.

The corresponding count noun is the noun phrase "data point" with plural "data points."

e.g.:

In a message dated Tue, 26 Feb 2002  4:32:49 PM Eastern Standard Time, "Steve Kl." <stevekl at PANIX.COM> writes:

> Just one data point, but there you go.

We do not say "Barry Popik has 15 data on 'Peeking Duck'" but rather "Barry Popik has 15 data points...".

"datum" now no longer means "data point" but rather has taken on the specialized meaning of "a data point of particular importance, often used as a reference from which other data points are computed."  The plural of "datum" in this new meaning is of course "datums".

e.g. Mead's Ranch is the datum, and all other latitude/longitudes on the Clarke 1866 spheroid are data computed from it.

or from aviation:  an airplane has a "reference datum", generally called simply "the datum".  This is an arbitrary point on the airplane, designated by the manufacturer, used for weight and balance computations.  (The use of a standard datum for each type of airplane simplifies the arithmetic.)

"Plane A and plane B have similar data" means that they have comparable wingspans, weights, speeds, ceilings, etc.

"Plane A and plane B have similar datums" is the correct usage if you want to say that A and B each has its reference datum at the same point, such as the propellor hub.

I rest my case, or to avoid mixing metaphors, I bring my case in for a landing.

      -James A. Landau
       Systems Engineer
       Federal Aviation Administration Technical Center (ACT-350/BCI)
       Atlantic City Airport NJ 08405 USA