Datums

Why '83 Was a Good Year for Datums

If it hasn't hit you yet, it will eventually. I'm talking about that curious little question: "What datum would you like that in?" And if you're like most people, you'll reply "Huh?" Actually, there's nothing to worry about. A datum is simply a numeric reference to a known set of points, which allow an accurate measurement to be assessed. Okay, so, I'm sure that clears things up completely.

No? All right, let me explain. As most of you are aware, the world is round (although there is a group of people out there who still think it's flat. But I digress.) Not to be the purveyor of bad news, but the world isn't really round. It's actually more elliptical in shape (a.k.a., a geoid). Furthermore, the surface of the earth is not smooth. There are those little things called mountains and valleys that seem to get in everyone's way. So when you place a point on the earth's surface and want to know what its latitude and longitude are, how do you account for these physical distortions?

Enter the datum. A long time ago, a group of people got together and came up with a known set of points that everyone else could use as a set of reference points from which to place new points. We'll call it the NAD 27 - Continental U.S. Datum. Then a few years later (56 to be exact), another group of guys got together and said, "You know, we've got better technology today. Let's come up with a new datum that's more accurate." So they did and called it the NAD 83 Datum. Unfortunately, there is a whole lot of data out there that was created using the NAD 27 standard. So, is it all wrong? No! It just uses a different standard of measurement. And since both of these datums, along with the hundreds of other datums I haven't mentioned, are simply sets of points that have a common denominator, namely the earth. It is fairly easy to translate from one set to another. In fact, if you use MapInfo®, it will automatically translate between the datums for you without you even knowing it.

Where's this trend going? NAD 83. And that's basically because the U.S. Census Bureau adopted this as its standard with the release of Tiger 95. So, now, when they ask you, "Which datum would you like that in?" you can intelligently reply, "83 was a good year!"

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