How to Use the Command 'gdal_contour' (with Examples)

The gdal_contour command is a powerful tool within the Geospatial Data Abstraction Library (GDAL) suite used for creating contour lines and polygons from a digital elevation model (DEM). Contours are essential for representing terrain elevation data visually and are widely utilized in geographic information systems (GIS) for various applications, including topography, land planning, and environmental analysis. This tool takes geospatial raster data, typically representing elevation, and generates vector data that can depict the landscape’s undulations and inclines.

Use Case 1: Create Contour Lines with Elevation Attributes

Code:

gdal_contour -a ele -i 100.0 path/to/input.tif path/to/output.gpkg

Motivation for Using the Example:

Creating contour lines from elevation data is a common requirement in geospatial analysis and mapping. Contour lines provide a simple, yet informative, visualization of terrain. Assigning elevation attributes to these lines ensures that each contour is labeled with its respective elevation value, making it easier for users to interpret the data, especially in applications such as printed maps or interactive GIS platforms where elevation detail is crucial for analysis.

Explanation for Every Argument in the Command:

• -a ele: This argument specifies that the contour lines will be attributed with the elevation data. The attribute field name “ele” is given, meaning that each contour vector will carry an elevation value under this attribute, adding significant value for users who require detailed topographic analysis.

• -i 100.0: The interval argument dictates the spacing between contour lines. In this case, lines are drawn every 100 meters of elevation, providing a moderate level of detail that is often balanced between readability and precision for large-area maps.

• path/to/input.tif: This is the input file, a raster dataset typically derived from a digital elevation model (DEM). The file is the source of the elevation data from which contour lines are generated.

• path/to/output.gpkg: This specifies the path and format for the output file. Here, the output is directed to a Geopackage format, which is a versatile and modern format that can handle spatial data efficiently.

Example Output:

A dataset containing vector contour lines spaced every 100 meters, each line tagged with its corresponding “ele” attribute. These lines can be integrated into GIS software, aiding users in visualizing and analyzing the gradients and topography of the landscape.

Use Case 2: Create Polygons Representing Elevation Zones

Code:

gdal_contour -i 100.0 -p path/to/input.tif path/to/output.gpkg

Motivation for Using the Example:

Creating polygons from elevation data is beneficial for applications requiring the delineation of specific elevation zones or bands, such as flood risk mapping, ecological zone analysis, and land use planning. Polygons define areas between contour lines and can be used to visually categorize regions based on elevation criteria, providing an expedient way to analyze elevation data beyond mere lines.

Explanation for Every Argument in the Command:

• -i 100.0: Similar to the contour line use case, this interval setting creates bands or zones with a 100-meter elevation difference between them. This choice of interval helps in creating discernible zones that capture sufficient percentage differences in elevation for analysis.

• -p: This argument instructs gdal_contour to generate polygons instead of lines. Polygons represent the areas between different contour intervals, offering a filled representation that is often more suitable for area-based analyses in GIS.

• path/to/input.tif: The raster input file containing the DEM data from which elevation zones are derived.

• path/to/output.gpkg: The specified output file format and location. Using a Geopackage aids in ensuring compatibility and scalability of the resultant vector data for various GIS applications.

Example Output:

A vector dataset where distinct polygons represent elevation zones within the raster data. Each polygon corresponds to an elevation band, facilitating analyses that require understanding of terrain areas falling within specific elevation thresholds.

Conclusion:

The gdal_contour command is a versatile tool for converting elevation data into meaningful visual representations in GIS. Whether generating contour lines with explicit elevation attributes or creating polygons to depict elevation zones, gdal_contour provides essential functionalities for any geospatial professional working with topographic data. As demonstrated in these use cases, users can tailor the command to suit their analysis needs, leveraging its powerful capabilities to enhance their spatial data interpretation.