How to use the command gdaldem (with examples)

The gdaldem command is a tool used to analyze and visualize digital elevation models (DEM). It provides useful functionalities to process and extract information from DEM datasets. This article will illustrate three common use cases of the gdaldem command: computing the hillshade, slope, and aspect of a DEM.

Use case 1: Compute the hillshade of a DEM

Code:

gdaldem hillshade path/to/input.tif path/to/output.tif

Motivation: Computing the hillshade of a DEM is useful for generating visually appealing representations of the terrain. It highlights the variations in elevation and creates shading effects that mimic the illumination from a light source.

Explanation:

• gdaldem: The command for running the gdaldem tool.
• hillshade: The specific operation to compute the hillshade.
• path/to/input.tif: The path to the input DEM file.
• path/to/output.tif: The path to save the output hillshade image.

Example output: The command will create a hillshade image file, which displays the terrain with shading based on the sun’s position and the angle of the slope.

Use case 2: Compute the slope of a DEM

Code:

gdaldem slope path/to/input.tif path/to/output.tif

Motivation: Computing the slope of a DEM helps in understanding the steepness of the terrain. This information is valuable for various applications such as hydrological modeling, land use planning, and geotechnical analysis.

Explanation:

• gdaldem: The command for running the gdaldem tool.
• slope: The specific operation to compute the slope.
• path/to/input.tif: The path to the input DEM file.
• path/to/output.tif: The path to save the output slope image.

Example output: The command will generate a slope image file, which represents the angle of inclination for each pixel in the DEM. Steeper areas will be depicted with brighter tones, while flatter areas will appear darker.

Use case 3: Compute the aspect of a DEM

Code:

gdaldem aspect path/to/input.tif path/to/output.tif

Motivation: Computing the aspect of a DEM provides information about the slope direction, indicating which way the terrain faces. This knowledge is useful for tasks like solar radiation modeling, landscape analysis, and understanding drainage patterns.

Explanation:

• gdaldem: The command for running the gdaldem tool.
• aspect: The specific operation to compute the aspect.
• path/to/input.tif: The path to the input DEM file.
• path/to/output.tif: The path to save the output aspect image.

Example output: The command will produce an aspect image file, where each pixel represents the compass direction (in degrees) towards which the terrain faces. Different colors are often used to visualize different directions.

Conclusion:

The gdaldem command is a versatile tool for analyzing and visualizing DEM datasets. It allows users to extract valuable information from elevation data. By understanding how to compute the hillshade, slope, and aspect, users can generate meaningful representations of the terrain and gain insights into the characteristics and behavior of the landscape.