DEM data: The world isn’t all black and white.

Lower Zion Canyon DEM 1/3"

In case you came in late this tutorial starts here

Digital Elevation Model data is a set of numbers which represent the elevations of various points on earth (and other planets too).  The resolution of the survey determines how closely the data points are spaced.

We usually visualize this data as a greyscale image with black the lowest point and white the highest.  This shows lower end of Zion Canyon with its surrounding plateau.

Over the course of 11 days in 2000 the Shuttle Radar Topography Mission (SRTM) obtained an elevation map of almost the whole earth.  The data has some holes in it so it is better to work with a cleaned copy rather than the raw data.

You can access worldwide data at http://www.imagico.de/map/demsearch.php .  The resolution is mostly 3 arc-second (or 3″ for short) which equates to about 90meter

resolution.  This is about the length of a soccer field.  At this resolution you can get a nice

STL model of Mt Everest made from 3" DEM data

model of mountains, but they aren’t as detailed as we might like.

For locations inside the United States there is much more accurate data in the form of the

AT&T Park DEM 1/9"

NED, the National Elevation Dataset.  The almost the whole country is mapped at 1/3″ or 10 meter resolution. Some parts of the country are available at an astonishing 1/9″

or 3 meter resolution.  At this resolution buildings can be seen. In this example the infield and even the pitcher’s mound at San Francisco’s AT&T Park is clearly visible.

For locations within the US the best source for data is the National Map .  The interface is a little cumbersome in that you must “order” data by placing it in your shopping cart and then entering your e-mail address and “checking out”.  There is no cost and you are e-mailed a link to download your data.  The e-mail can be instantaneous or several minutes delayed depending on server load.

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