How to use the command 'gdalbuildvrt' (with examples)
The gdalbuildvrt command is a utility within the Geospatial Data Abstraction Library (GDAL) suite of tools. It is specifically designed to build virtual datasets, known as VRTs, from a list of existing raster datasets. By creating VRTs, users can efficiently manage and process large collections of geospatial data without the need for extensive disk space typically required for traditional file mosaicking. A VRT acts as a pointer to the existing data while enabling operations such as mosaicking, subsetting, or stacking multiple bands into one composite dataset.
Use case 1: Making a virtual mosaic from all TIFF files contained in a directory
Code:
gdalbuildvrt path/to/output.vrt path/to/input_directory/*.tif
Motivation:
Creating a mosaic is a common task when handling multiple remote sensing images or topographic datasets that cover adjacent areas. Instead of working with each individual file separately, combining them into one cohesive dataset helps streamline data analysis and visualization. This command allows users to efficiently build this mosaic virtually, which saves disk space since a VRT file points to the original data rather than duplicating it.
Explanation:
gdalbuildvrt: The base command used for building virtual datasets.path/to/output.vrt: Specifies the path and filename for the resulting virtual dataset.path/to/input_directory/*.tif: Uses a wildcard to specify all TIFF files located in the target input directory. This tells the program to include each file in the directory as part of the mosaic.
Example Output:
The command produces a single VRT file at the specified output path. This VRT file acts as a virtual blend of all TIFF images in the input directory, which can be opened and viewed as a single seamless image in any GDAL-compatible software.
Use case 2: Making a virtual mosaic from files whose name is specified in a text file
Code:
gdalbuildvrt -input_file_list path/to/list.txt path/to/output.vrt
Motivation:
In scenarios where only specific geospatial files need to be included in a mosaic, using a text file to list these filenames provides precision and flexibility. This is especially useful for large folder directories containing files of various formats or for batch processing selected datasets without manually typing each filename into the command line.
Explanation:
gdalbuildvrt: Initiates the virtual dataset creation process.-input_file_list: An option indicating that the input filenames are to be specified via a text file.path/to/list.txt: Path to the text file which contains a list of filenames. Each line in the file should contain one filename to be included in the mosaic.path/to/output.vrt: The path and filename for the final virtual dataset file.
Example Output:
The output is a virtual dataset file that consolidates only the images listed in list.txt, producing a VRT file that represents those specific files as one cohesive mosaic.
Use case 3: Making an RGB virtual mosaic from 3 single-band input files
Code:
gdalbuildvrt -separate path/to/rgb.vrt path/to/red.tif path/to/green.tif path/to/blue.tif
Motivation:
Individual band files are often generated from multispectral sensors due to separate storage for each electromagnetic spectrum band. Merging these single bands into an RGB composite allows for traditional true-color visualization, which is critical for applications like visual inspections and initial analyses. This command accommodates this need without duplicating data, creating a virtual RGB image.
Explanation:
gdalbuildvrt: The command to initialize VRT creation.-separate: An option that indicates the input files should be treated as separate bands to form a multiband VRT.path/to/rgb.vrt: Specifies where the output virtual dataset will be saved.path/to/red.tif,path/to/green.tif,path/to/blue.tif: These are the input single-band images for the red, green, and blue channels, respectively.
Example Output:
The output consists of a VRT representing an RGB image by stacking the specified red, green, and blue bands into a single virtual file. This allows users to handle and visualize them as a single composite RGB image.
Use case 4: Making a virtual mosaic with a blue background color (RGB: 0 0 255)
Code:
gdalbuildvrt -hidenodata -vrtnodata "0 0 255" path/to/output.vrt path/to/input_directory/*.tif
Motivation:
Geospatial datasets often contain nodata values representing missing or undefined data. Setting a distinct background color for these nodata values not only visually separates them from valid data but also enhances readability and presentation in map products. A blue background is a popular choice since it contrasts well with natural or terrestrial data.
Explanation:
gdalbuildvrt: The main command for creating a VRT.-hidenodata: This option tells the program to hide nodata values in the output VRT.-vrtnodata "0 0 255": Specifies that the nodata areas should be represented by the color blue (RGB value of 0 0 255) in the VRT.path/to/output.vrt: Determines the destination of the virtual output file.path/to/input_directory/*.tif: Uses a wildcard to select all TIFF files in the input directory for the mosaic.
Example Output:
The operation results in a VRT file with all input TIFF files mosaicked. Nodata areas are visually filled with blue in the output, making these regions easily distinguishable for further analysis or presentation.
Conclusion:
The gdalbuildvrt command is a powerful tool for efficiently managing and showcasing geospatial raster data. By facilitating the creation of virtual mosaics and enabling diverse customization options, it supports a wide range of use-cases pertinent to geomatics professionals, from remote sensing analysts to GIS specialists. Through its flexibility and avoidance of data duplication, gdalbuildvrt ensures high compatibility across data types, with optimal storage use and customizable visualization options.
