What’s a datum?

Earlier in the week I gave a quick overview of map projections. Today I’m covering a related topic–datums. North American Datum 1983 is the basis for a lot of common map projections, and World Geodetic System 1984 is the series of latitude and longitude most commonly used on consumer GPS devices.

But first, what’s a datum, and how does it relate to map projections? A datum is one way we can reference positions on Earth’s surface. Since the planet is not a perfect sphere or even a perfect ellipsoid, a datum needs to define the shape it’s assuming the Earth holds. A datum is defined by the ellipsoid shape it uses and the lengths of the major and minor axis. When you see coordinate data (usually in latitude and longitude, but sometimes in other units), you’re viewing unprojected data that references a datum. Those coordinates can then be projected so that you can make a flat map. When you’re using a projected map, it’s useful to know both the projection and the datum used.

NAD83 is the current formally accepted datum for North America. We’ve had two previous noteworthy datums here– United States Standard Datum in 1901 and the North American Datum of 1927. These two earlier datums are very similar and are based off of the same assumption regarding the shape of Earth (Earth is not a perfect sphere, so to map it we need to approximate it’s ellipsoid shape). NAD83 is based off of a newer, more accurate ellipsoid shape.

datum_shift_between_nad27_and_nad83Some people still use NAD27, but most people use NAD83 or a variant. Interestingly, NAD27 and NAD83 are basically the same in the Great Lakes region (see picture to the left, from here). But, if you live in the western part of the continent you can be off by nearly 100 meters. Fortunately, there is an easy to use converter available from NOAA here.

WGS84 is another very common datum. WGS84 uses its own reference ellipsoid, and has been based off of GPS observations since the mid 1990s (previously it had been using Doppler observations). Since it is using GPS observations, which utilizes moving satellites, it can drift slowly over time. The current WGS84 can vary from NAD83 by up to 2 meters, but for most user applications this is not noticeable, especially in the US.

NOAA has a really detailed description of many NAD datums (yes, there are more than the three I briefly described above) and the history of WGS84. If you plan on using coordinate data from before the 1980s for any purpose, reading up on the details of each datum can help you best convert information and control for error.

Coming up for you this month I have an additional North American Datum to share, and some information about different projections used to map NAD information. I’ll talk about how I pick a projection for analysis, how to share projection information between map users, and other practical guidelines for spatial data handling.


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