When using a vertical datum, you can describe a point as being x-feet above or below the “Mean Sea Level (MSL).” Vertical datums use the surface of a geoid model to establish a zero-point of elevation. While horizontal datums pinpoint a location on the earth’s surface, a vertical datum gets you elevation data. Vertical datums: How low (or high) can you go? And while horizontal measurements between the WGS 84 and NAD 83 are on average only one meter, that’s a big enough difference in some disciplines-digging a trench on a construction site or creating a haul road on a mining site, for instance-to make a datum transformation from GPS collected coordinates to your local datum necessary. The center of the WGS 84 ellipsoid is generally considered to be about 2cm off the center of the earth’s mass.Īll GPS data is referenced through the WGS 84. The WGS 84 is an example of a geocentric datum. With the advent of GPS, geodesists were able to develop a highly accurate worldwide ellipsoid model and corresponding datum called the WGS 84 that is still used for global mapping purposes today. Over the past 37 years, the NAD 83 has been updated several times thanks to advances in GPS and geodesy. It also used terrestrial and Doppler satellite data to correct some of the distortions in distance in the NAD 27. The NAD 83 datum expanded on the NAD 27 to over 250,000 stations. Over the years, more and more stations were benchmarked. Unlike today’s ellipsoids, which use their geometric center as the origin for all coordinates, the Clarke Ellipsoid used a point in Meade’s Ranch, Kansas, which was approximated to be in the center of the contiguous United States. The NAD 27 datum used the Clarke Ellipsoid of 1866 as its basis. To get to the NAD 83, geodesists and surveyors first had to establish the NAD 27, which took decades of triangulating the relationships between tens of thousands of survey monuments all around the country. If you were to use NAD 83 anywhere outside North America, your measurements would be off. In other words, you want a local datum.įor instance, in the United States, surveyors use the North American Datum 1983 (NAD 83), which is based on the Geodetic Reference System 1980 (GRS 80) ellipsoid. Knowing this, when surveying, you ideally want to use the datum whose ellipsoidal model best fits your area. Otherwise, the same points in the real world will be assigned different points on your map, and your measurements will be off.Īs we just established, the Earth’s surface is not uniform. When comparing survey datasets, they need to be referenced through the same datums. Once the ellipsoid is “pinned” to the earth, we can use the horizontal datum to start assigning angular units of longitude and latitude to different points on the earth’s surface. ![]() ![]() Horizontal datums take an ellipsoid and assign its center a point of origin relative to the center of the earth. To be useful, we also need to know which specific model to use and where to place them in space. But that only gets us halfway to our end goal. So, ellipsoids and geoids define the size and shape of the earth, to varying degrees of accuracy. Horizontal datums: finding your place in the world ![]() These bumps and dips on the earth’s surface are a result of the planet’s irregular density, which causes gravity to be stronger in some places than others. Geodests and surveyors use ellipsoids to assign points angular coordinates, or degrees of longitude and latitude.Ī geoid is a more dynamic model of the earth that accounts for undulations in the planet’s surface. Unlike the earth, it has a smooth, uninterrupted surface.Įllipsoids are defined by their geometric parameters, which include a semimajor axis (the radius of its equator) and semiminor axis (distance from its center to either pole). Like the earth, it’s spherical, but not perfectly so. (You can check out a more thorough explanation of these topics here, but in short:Ī reference ellipsoid is an approximated, more uniform model of the earth. To understand the purpose horizontal and vertical datums serve, we must first discuss two related concepts: ellipsoids and geoids. And for a deeper dive into topics related to CRSs and geodesy, you can download our ebook on the subject: Ellipsoids and geoids Read on for a quick explanation of horizontal and vertical datums, how they’re used in surveying, and how they’ve evolved over the years. Because understanding the function of datums is crucial to getting the bigger picture of CRSs, we created this quick primer. In fact, the term “datum” is often used interchangeably with “ geographic coordinate system” (or a type of CRS used for specifying the location of a point on the earth).īut a datum is only one component of a geographic coordinate system-albeit an indispensable one. Of the elements that make up a coordinate reference system (CRS), none are more fundamental than datums.
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