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<!DOCTYPE html>
<html>
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<title>map school: The Shape of the Earth: a free introduction to geo</title>
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<h1><a href='./'>map school: The Shape of the Earth</a></h1>
<p>a free introduction to geo.</p>
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<p><img src="img/earth-shapes.jpg" alt=""></p>
<p>Storing and presenting the world brings us to the question of what shape it is - can latitude and longitude values be mapped to a perfect sphere and back, and retain their accuracy spatially?</p>
<p>Since the earth is a spinning object and its components can change shape, it bulges at the middle - so instead of being a sphere, it's actually more similar in shape to a <a href="http://en.wikipedia.org/wiki/Oblate_spheroid">oblate spheroid</a>. If you look even closer, that isn't entirely true either - the earth is covered in <a href="http://en.wikipedia.org/wiki/Topography">elevation differences</a> like mountains and valleys, and even manmade changes like cities.</p>
<p>For day-to-day work, we use different estimates of this shape: standards like <a href="http://en.wikipedia.org/wiki/WGS84">WGS84</a> define precise values for the length of both of the earth's axes, so we can estimate for a <a href="http://en.wikipedia.org/wiki/Reference_ellipsoid">reference ellipsoid</a> rather than a sphere. Local measurements and science that relies on precise surface values can also use <a href="https://en.wikipedia.org/wiki/Geoid">geoid models</a>, which are three-dimensional calculations of theoretical ocean heights.</p>
<p>This branch of earth sciences, called <a href="https://en.wikipedia.org/wiki/Geodesy">geodesy</a>, is a continuing project since our ability to model and measure the earth moves quickly and the earth itself changes in shape.</p>
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