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<!DOCTYPE html>
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<title>map school: Data Collection: a free introduction to geo</title>
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<h1><a href='./'>map school: Data Collection</a></h1>
<p>a free introduction to geo.</p>
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<p>Map data has been collected in countless ways through the years - everything from sailors' logs to geocoded Tweets. Currently, there are a few major sources that merit discussion:</p>
<p><img src="img/gps.jpg" alt=""></p>
<p><strong>GPS</strong>, the satellite constellation that gives your cell phone a blue dot on the map, is the foundation of collecting accurate vector data. Surveyors will drive with highly accurate GPS units and combine their results into something trustworthy.</p>
<p>Observational <strong>satellites and airplanes</strong> collect most raster data we have today, constantly taking photos from different altitudes and combining them into something that looks a little like a picture of the world. The same sensors also capture what we call 'non-visible spectrums', like infrared light that's useful for mapping agriculture. Some tricked-out rigs include <a href="http://en.wikipedia.org/wiki/Lidar">LiDAR</a>, a kind of laser sensor that measures altitude and yields us raster altitude data.</p>
<p><strong>Corporations, governments, and communities</strong> maintain different world maps of varying detail. For instance, Google and <a href="http://www.openstreetmap.org/">OpenStreetMap</a> focus on mapping all roads and details about them, and sources like the <a href="https://www.cia.gov/library/publications/the-world-factbook/">CIA World Factbook</a> and <a href="http://www.naturalearthdata.com/">Natural Earth</a> keep track of political borders.</p>
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