QGIS: The open source GIS Swiss Army knife

In case you came in late this tutorial starts here

The first step in processing the DEM data is to import it into QGIS.  QGIS is a fairly imposing program which can do anything to GIS data except show you how to fold the map back the way it came.  I will show the process step by step so we don’t get lost in its murkier depths which are crawling with pythons (actually written in python).

After you have QGIS open and a blank project staring you in the face click the checkerboard Add Raster Layer button you will be prompted to select your DEM data file to import.  DEM data downloads consist of a zipped folder with a dozen or so files.  One file is huge compared to the others.  This is the actual data so import this file.  The other files are important and must be in place in the folder because they tell QGIS how to

interpret the data and where to locate

Clipping out the area of interest using QGIS

it.  We will use the Raster>Extraction>Clipper tool to trim our data.  You can enter coordinates or click and drag a box around the area you want to extract.  The numerical entry method is necessary for making collectible mountain models, but usually I just click and drag framing the area I want for the model.

When you name the output file be sure to add the extension .tif to the name.  QGIS doesn’t care, but 3DEM won’t recognize a file without the extension.  After you extract the area it is automatically saved as a GeoTiff with the name and location you specified.  Notice how the extracted areas have higher contrast than the rest of the image.  The DEM data is a 32bit greyscale file, but your eyes, aren’t up to the task of discerning all that depth.  QGIS renders the image with the greyscale values scaled to the selected area so the selected areas are much clearer.  If you right click on the raster layer’s name and select properties you can manually change the rendering of the image.

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