If you do that, you wind up with what we call a geoid. Imagine flattening all topography on Earth straight down to sea level. So what's the deal with mountains? The short answer is - for lat / long mapping of the Earth, we essentially just pretend that all mountains are flat. In all of the above, we didn't talk about topography hardly at all. Once you have a datum established, which is a locked in place, smooth spheroid 3D model of a bulging unsmooth real Earth, you can now assign real world locations to latitude and longitude coordinates on the datum / spheroid model. The most common global-fit datum is the 1984 World Geodetic System, commonly referred to as WGS84. The most common North American best-fit spheroid is the 1983 North American Datum, commonly referred to as NAD83. Once we've made a best-fit spheroid choice, we call it a datum. For example, here's a comparison of two spheroids designed to best-fit North America and Europe. We're forced to make a tradeoff between a global spheroid that's only somewhat accurate, but equally poor-fitting everywhere, versus spheroids that better fit a particular region at the expense of much worse accuracy on the other side of the globe. ![]() No single spheroid fits all points on Earth best. Before we can start mapping real Earth locations onto our spheroid, we need to decide where to position our spheroid in 3D space. And we use spheroids (a special kind of ellipsoid) to model that shape. Okay, so we've figured out the Earth is lumpy and bulged at the middle. If all three axis lengths are equal, they all therefore must be circles and you get a sphere.Įarth bulges only in the middle, so the smooth surface we use to model it is a spheroid, which is really just a special kind of ellipsoid. If two of those lengths are equal, then you get a one circular axis and one bulge or thin axis. An ellipsoid is a round, smooth surface where each of the three axes may be different lengths. You know how squares can be defined as a special kind of rectangle where both sides are equal? Well, use the same principle for round shapes in 3D and what we have is:ĭifference between ellipsoid, spheroid, and sphere Many geographers use these terms interchangeably but there's a difference. This is where spheroids and ellipsoids come into play. We want a smooth surface which bulges a bit in the middle. What shape? Remember that the Earth is fatter at the equator, so a perfect sphere doesn't fit very well. To make a consistent map, we first smooth out the lumps of the Earth and map them to a smooth shape. ![]() Here's a very over-exaggerated true shape of the Earth. Then the tectonic plates and other effects create gravity wells. It's fatter around the middle because rotation makes bulges at the equator. There are several concepts that take us from an imperfect, lumpy Earth to an x-y 2D map. So, before answering OP's question, thought I'd go ahead and make sure everyone in this thread truly understands what a datum even is. So, lat/long is a coordinate system we use for mapping location on datums. A coordinate system is any type of measuring system used to map space on either 2D or 3D surfaces. We use lat/long angular coordinates to map location on sphere-like objects. Tl dr Datums are 3-dimensional smooth spheroid models of Earth.
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