Working with Elevation data in QGIS

The information which can be derived from Elevation data have a lot of applications in different fields of the science, from hydrology, risk analysis, urban planning among others. In this tutorial and in the following, we will learn the basics of DEM manipulation in QGIS in order to derive useful information which can be used with different purposes. The aim of these tutorials is not to provide with deep explanations about the methodology (algorithms) itself but rather give general ideas about the available tools to perform those analysis. As introduction will learn how to derive the slope and hillshade using QGIS.

One of the most useful products which can be derived from a DEM is the Slope map. The slope measures the steepness or degree of inclination of a feature relative to the horizontal plane. The slope can be measured as percentage, angle or as a ratio.

A very nice and detailed description can be found here: Slope Calculation from Contour Lines in a Topographic Map

Note: Is it usual to confuse or misinterpret the slope values when using degrees and when using percentage. Remember, a 100% slope does not mean total verticality, it means 45 degrees of inclination, geometrically is completely possible to have slopes higher than 100% since it means that the terrain’s inclination is higher than 45 degrees. In the other hand when using degrees the representable range of slopes cannot go further than 90 degrees.

QGIS offers several options to derive this parameter, I will show you two of them (which I had used in the past). The first option is to use the Slope command located in the Terrain analysis in the Raster tab. This plugin is installed by default in the normal installation of QGIS (I use QGIS 2.14).

This plugin allows the user to calculate the slope, aspect, hillshade model, relief and Ruggedness Index. In this case the slope is calculated using the first order derivative estimation (QGIS 2.8 documentation).

If we select the slope option, the following dialog box will be shown:

The application of the algorithm is very straightforward, first, we have to select the raster layer with the altitudinal information (DEM) and then write down a name for the slope layer to be generated. We can also choose between several formats (I normally use tiff unless I need the generated file to be used in other specific application).

With regard to the z-factor, it is used when the units of the X, Y and Z units are not the same, by default a value of 1.0 it means that the units of the three axis are the same.

After these steps the slope layer is generated, remember, this tool generates a slope map in DEGREES.

As you realized, the previous tool is very easy to apply but it does not offer a lot of options to configure the output (change of units for example). In this case, QGIS offers several additional options to derive the slope with different methods, formats and also units. I will show you the Slope, aspect, curvature tool which is part of the SAGA GIS tools.

This tool is located in the Processing toolbox.

This tool requires as input the DEM and can generate several derived products (Aspect, curvature…..) aside from the slope layer.

If you do not want all the layers the algorithm can generate, you can unselect them form the list.

When all the required output files are completed, the algorithm can be run and it will create the selected layers in the selected locations.

Conclusion and tips.

• QGIS offers several options in order to derive the slope and other DEM-related products (aspect, curvature……), at least 4 tools can be used to obtain DEM-derived products in QGIS: the included QGIS algorithm, the SAGA GIS tools, the GRASS GIS tools (r.slope and r.slope.aspect).
• The products slightly differ among them and there is not a unique answer, the election of the adequate tool depends on several factors, like units, formats and algorithms.
• This tutorial only shows the basics of the tools without deepening in details, if more information is required, the documentation of each tool might be very useful.
• Sometimes the tools might encounter problems with long paths and names with spaces or special characters, try to avoid them.

Written by: Marlon Calispa