I think the use of "data" as a singular, collective noun mostly stems from the fact that its singular form ("datum") is a nearly useless word. The word "data" has been thoroughly bound to the context of statistics and its applications: science, engineering, economics, etc. These are fields which generally discourage using single points of data or even small sets of data. As a matter of course, people work with a plurality of data almost exclusively. It's rare that person needs to refer to a single datum at all. In that rare case, phrases like "point of data" or "piece of data" suffice.
To make matters worse, in many if not most cases it isn't even very clear exactly what constitutes a single "datum". Case in point, the first thing that pops into my head when you say "datum" is the
North American Vertical Datum of 1988http://tidesandcurrents.noaa.gov/datum_options.htmlQuote:
The National Geodetic Survey (NGS) defines a geodetic datum as: 1. "A set of constants used for calculating the coordinates of points on the Earth." Generally a datum is a reference from which measurements are made. In surveying and geodesy, a datum is a reference point on the earth's surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude. Vertical datums are used to measure elevations or underwater depths.
North American Vertical Datum of 1988 (NAVD 88)
A fixed reference for elevations determined by geodetic leveling. The datum was derived from a general adjustment of the first-order terrestrial leveling nets of the United States, Canada, and Mexico. In the adjustment, only the height of the primary tidal bench mark, referenced to the International Great Lakes Datum of 1985 (IGLD 85) local mean sea level height value, at Father Point, Rimouski, Quebec, Canada was held fixed, thus providing minimum constraint. NAVD 88 and IGLD 85 are identical. However, NAVD 88 bench mark values are given in Helmert orthometric height units while IGLD 85 values are in dynamic heights. See International Great Lakes Datum of 1985, National Geodetic Vertical Datum of 1929, and geopotential difference. NAVD 88 should not be used as Mean Sea Level.
A "datum" is a "set of constants" derived from "terrestrial leveling nets"? Yeah...