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section-datum.html
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<!DOCTYPE html>
<html>
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<meta charset="utf-8">
<title>map school: Datum</title>
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<meta name="description" content="a brief introduction to map and geospatial concepts">
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<div class='header'>
<h1><a href='./'>map school: Datum</a></h1>
<p>a free <a href='https://github.com/tmcw/mapschool'>open</a> introduction
to geo.</p>
</div>
<a class='feedback' href='https://tmcw.wufoo.com/forms/mapschool-feedback/' target='_blank'>feedback</a>
<div class='wrap'>
<div class='content'>
<p>A geographic datum is a specific way of measuring places on earth. Back in the day, this was a hard problem, since surveyors would require a high level of accuracy for specific areas: measurements down to the inch for a town. And so, instead of just saying latitude, longitude, we'd say "meters from this spot measured North/South."</p>
<p>Practicing cartographers don't need to know much about the internal operations of datums anymore, since it's handled in software and most special formats can be automatically translated. A global standard called WGS84 caught on for general-purpose mapping and is the recommended way to store data.</p>
<p>Though you don't often need to know the details of transforming between datums, when working with geo data from different sources it is <em>very</em> important that you confirm that the datums match. Working with data in different map datums can result in non-trivial errors, particularly at small scales on the order of 0-200m.</p>
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