What’s a datum?
If you were asked to determine the precise latitude and longitude of a specific location, say the spot where you are sitting right now, how would you go about that? Buying a GPS unit is the obvious response today, but that option wasn’t always available. How was it done in the "old" days?
To accomplish this task, as is routine for surveyors, you simply start at a specific location for which the latitude and longitude is already known, i.e. a "know" point. From this point, a traverse is made to the new point. That is, precise measurements are made about the distance and direction of the new point relative to the known point. Using these measurements and perform some complicated calculations, and finally you have the desired geographic coordinates for the new point.
How were the latitude and longitude of the "known point" determined, you might (hopefully) ask? The answer is by using the same procedure. That is, start at a known point, measure, and calculate. Of course, sooner or later you get back to a single "known point", i.e. the mother of all known points. This is what is actually referred to as a datum, i.e. the singular form of the word data. How were the geographic coordinates of this point determined?
One way this could have happened is that a group of people could have set up camp at a place, let’s say a place known as Meade’s Ranch, Kansas; approximately the geographic center of the 48 states. And while there, they used telescopes, pendulums, gravity meters, or whatever it is that those folks use (they’re called geodesists), to determine the latitude and longitude of a single point, and the true azimuth to another point. This combination is called, in the geodetic sense, a datum. This is the absolute minimum information necessary to determine what the location of the second point is using more traditional techniques. Thus, the determination of the latitude and longitude of the second point was, relatively, easy. A matter of measuring and calculating. Given two known points, and using triangulation, measuring, and calculating, additional "known points" were established using rather traditional surveying techniques.
Before long, a whole network of known points, all referenced to the original datum were established which could be used by surveyors all over the country to perform their tasks. In fact, some 50,000 point were to be originally established While doing so, definitive information about all measurements made, calculations performed, and results obtained were carefully recorded. The end result is a well defined network of 50,000 "known" points. The technical name of such a network of known points is a Horizontal Geodetic Reference System. Local Geodetic System and Geodetic Reference System are also used. However, most folks simply use the term "datum" to refer to this network. This leads to the terminology to which you may have seen (or will be seeing now that your a casual cartographer), namely the North American Datum of 1927 (i.e. NAD27) and/or the North American Datum of 1983 (NAD83).
Why is there more than one datum? That’s a story of its own which we’ll cover next time. Some liberties with actual historical fact were taken to produce the above description of the development of the North American Datum of 1927. The development of a datum is a labor intensive and expensive proposition, so the development of a new datum is almost always based on the work, carefully documented as described above, of any previously existing datum. Thus, the North American Datum of 1927 was actually based on work previous. The real "mother of all known points" was established in 1879, is located in Maryland, and is appropriately named Principio. However, it is assumed that all that occurred between 1879 and 1927 to produce the North American Datum of 1927 is a bit more than the Casual Cartographer needs, or wants, to know.