{ "id": "2000-march-the-worlds-not-flat", "slug": "2000-march-the-worlds-not-flat", "url": "https://dwillis.github.io/uplink/article/2000-march-the-worlds-not-flat.html", "year": 2000, "month": "March", "monthNum": 3, "headline": "The world's not flat", "kicker": "MAPPING", "author_name": "Andy Lehren", "author_title": "", "affiliation": "Dateline NBC", "summary": "The article explains that while the Earth is round, determining its exact shape is complex, involving different geodetic measurements and datums that affect how maps represent locations on our planet.", "keywords": [ "mapping", "datum", "ellipsoid" ], "topics": [ "Technology", "Investigative Methods" ], "technologies": [ "GIS" ], "full_text": "The world's not flat\n\nBy Andy Lehren Dateline NBC\n\nIt sounds obvious: the world is round. Aristotle knew that back around 350 B.C. Ptolemy refined it a few hundred years later. Despite fears from the flat earth believers, Columbus made a living off the idea.\n\nThe problem is knowing the exact shape of our sphere.\n\nYou don't have be sailing for a new world to face the question. All you have to do is get a few disparate maps and try to make them work in your GIS program.\n\nThere are two fundamental questions to ask when you get a map from anyone: what's the datum, and what's the projection. There are other details that may be helpful, but these are the most important. This column will try to explain datum, and how to convert it when you have to.\n\nSome more history: Isaac Newton wrote in the late 1600s that, well, the earth's a bit flattened on the top and bottom. The French, after measuring their country around the same time, countered that it's more egg shaped. Newton won the debate. The French ate better food. And we live with the question: just how flat on the top and bottom?\n\nEver since, geodesists have been measuring this. There are hundreds of calculations that have been done figuring out our little ellipsoid. Here are the three that matter:\n\nClarke 1866, developed by Alexander Ross Clarke. He was a busy guy, so the year is important.\nGeodetic Reference System 80 (GRS 80), a group effort by an international geodetic organization.\nWorld Geodetic System 84 (WGS 84), founded by the U.S. military and close to GRS 80, but not exactly the same.\n\nThese different measurements of our spinning ellipsoid are the underpinnings for almost all the maps we look at. You can tell from the history that this is still evolving, and you may well face other datums as you do more mapping.\n\nYou May Not Be Where Your Map Says You Are\n\nYou would think that something like latitude and longitude is absolute. It's not. First, different geodesists came out with different calculations on how elliptical is our planet. If you try to draw three circles by hand, you'll have an idea of the differences.\n\nThese ellipsoid measurements are the underpinnings for datum. You may be most familiar with datum by seeing latitudes and longitudes, though it's more than that. If geodesists draw up different spheres, the datum will be different. It gets murkier. To develop datum, geographers went out and mapped our country. They did not measure every latitude and longitude. They measured a lot, and extrapolated a lot more. They used the ellipsoid calculations as their reference guides for figuring datum. In other words, there's some guess work piled on top of some guess work. Early this century, geographers used the Clarke 1866 ellipsoid, and result was something called North American Datum 1927 (NAD 27), with the center in Meades Ranch, Kansas. It had problems. They measured again later that century. This time they assumed the Earth was shaped more like the GRS 80 ellipsoid, and came out with something called North American Datum 1983 (NAD 83). And now people are running around with GPS units measuring where they are, and assuming the Earth is shaped like the WGS 84 ellipsoid. That's how we've gotten the three major kinds of datum that are the foundations for the maps we use. Three different ellipsoids were used to derive three sets of datum. And that means three sets of latitudes and longitudes. You pull out a GPS, and it can tell you your latitude and longitude. Well, not exactly. It says where you're at based on WGS 84. That's the flavor preferred by most GPS devices. That latitude and longitude may be off by half a meter or more using GRS 80. And it may be up to 300 meters off under Clarke 1866. If you don't know the background of your map, you could use a GPS, look at a map, and realize you may not be exactly where you think you are.\n\nConverting Your Maps\n\nOK, sometimes your maps will be in NAD 27. And sometimes they are in NAD 83. And when you use your GPS, you're in WGS 84.\n\nIn a perfect world, you would try to work in the datum used to make your maps. But that doesn't always happen. You get a map from one agency in one datum. You want to match it against a map in a different datum. Now you need to run a conversion so they line up better.\n\nHere are a couple solutions. These examples are for converting datum only. (Just so you know, the ArcView utility mentioned here can handle projection conversions. There is also a script in older versions of ArcView to switch projections.)\n\n1. ArcView's Projection Utility\nThe new version of ArcView comes with something called the Projection Utility. You can run it outside the program. Just click on the icon on your desktop. Once you're in the utility, point and click your way to the shape file you want to convert. Hit the next button. That ends step one. For step two, first check the box that says \"Show Advanced Options.\" We're assuming the data is not projected, so keep the coordinate system at \"Geographic.\" Choose the datum your map came in, say North American 1927. Hit the next key. For step three, select the datum you want to convert to, like North American 1983. Hit next. Step four is where you navigate to where you want your converted files. Now you can hit the next key and then finish. The utility will create a reference file that keeps track of the datum for your new file.\n\nThe new ArcView program is based on code written by a group of oil and gas companies. In its manuals, ESRI, the maker of ArcView, does not talk about errors and estimations in running conversions. But the program is not perfect. The petroleum consortium whose work is behind the ArcView utility acknowledges that fact. Your end result may be off by a meter or more.\n\n2. Wessex' TransNAD\nIf you have an older version of ArcView, or use MapInfo, here's another solution that's free. Go to www.wessex.com. It's the home page for a company that sells street map data and other things.\n\nFrom the home page, navigate to utilities, and download TransNAD. It's a GUI application that lets you point and click your way quickly to a solution. It only lets you convert between NAD 27 and NAD 83. Unlike the new ArcView utility, this program will not make a new file, but alter your original file. So it's best to work off a copy.\n\nOnce you've installed the program, navigate to the file you want to convert. Choose which conversion you need to run, either from NAD 27 to NAD 83, or visa versa. Hit next to run the conversion. You're done.\n\nAndy Lehren can be reached by e-mail at alehren@nbc.com", "provenance": { "extracted_by": "anthropic/claude-sonnet-4-6", "extracted_at": "2026-07-05", "verified": true }, "similar": [ { "slug": "2000-june-the-right-projection", "score": 0.119 }, { "slug": "2000-november-using-subqueries", "score": 0.099 }, { "slug": "1998-october-car-covered-planet", "score": 0.097 }, { "slug": "1998-august-tech-tip", "score": 0.083 }, { "slug": "2000-november-web-links", "score": 0.078 } ], "html_url": "https://dwillis.github.io/uplink/article/2000-march-the-worlds-not-flat.html", "json_url": "https://dwillis.github.io/uplink/article/2000-march-the-worlds-not-flat.json" }