Python Scripting Options For Clipping Raster Data In Qgis

The Problem of Large Raster Files

Handling large raster datasets can be computationally intensive for desktop Geographic Information Systems (GIS) software. Raster files containing continuous gridded data, such as digital elevation models (DEMs), satellite imagery, and aerial photos, can occupy substantial disk space. For example, high-resolution LiDAR-derived DEMs for modeling terrain and hydrology across landscapes often require multiple gigabytes per tile. Analyzing raster data across large regions involves loading and processing abundant cells in memory and can slow workflows.

Clipping rasters to smaller subsets makes data more manageable for analysis and storage. This allows focusing computational effort on specific Areas of Interest (AOIs) relevant to a study area or project site. Reducing unneeded raster data volume enables faster visualization, terrain processing, hydrological modeling, and spatial analysis in GIS software. Raster clipping creates smaller exported files that integrate better with reports and field data collection.

Leveraging Python and GDAL for Efficient Clipping

Python scripting harnesses advanced programming capabilities for automating and enhancing GIS workflows in QGIS. Python integration leverages Geoalgorithms and the GDAL/OGR libraries for productive raster processing. The GDAL binding provides interfaces for clipping, reprojecting, and exporting raster datasets using optimized algorithms fine-tuned for performance.

Table of Contents

Automating raster manipulation steps with Python reduces manual repetition, cuts overall processing time, and applies custom parameters across datasets. Python enables batch clipping many rasters to AOIs in a single run. Users avoid having to clip rasters one by one through graphical user interfaces. Programmatic access to GDAL/OGR tools enables scenarios hard to replicate manually, like clipping hundreds of rasters based on vector cutlines with a single script.

Key Tools and Functions

The gdal.Warp() function executes GDAL raster clipping and exporting capabilities through Python. It requires input rasters, output clipped raster paths and filenames, and cutline vectors or bounding boxes as clip shapes. The function applies reprojection, cropping, and filtering during execution per specified parameters.

Vector layers with polygon features define clipping boundaries based on their shapes and locations. Converting features in an existing QGIS vector to polygons provides ready cutlines. The QgsGeometry class manipulates vector geometries, enabling constructing polygons from scratch in scripts. The ogr module interfaces with OGR vector sources for input and output.

Clipping against target spatial references or projections provides control over output raster coordinate systems during gdal.Warp() execution. The function matches cell sizes between input and output grids by default. Optional resampling algorithms handle remapping raster values when cell sizes change.

Walkthrough: Clipping a DEM to a Watershed Boundary

This walkthrough demonstrates a workflow for clipping a raster DEM to the boundary of a vector watershed layer. It focuses on isolating a hydrological DEM subset for more efficient watershed-scale analysis.

First, load the input DEM raster and vector watershed layer into QGIS. Ensure their Coordinate Reference System (CRS) matches to overlay properly. If unmatched, reproject either to match CRS using gdal.Warp() or QGIS reprojection tools. Confirm correct watershed overlay on the DEM for quality control.

With layers overlaid, create the polygon cutline from the watershed vector using the QgsGeometry class. The code extracts the feature geometry as a multipolygon then casts to a single polygon object. This becomes the cutline passed to gdal.Warp() for clipping the area it encloses from the DEM.

Setup gdal.Warp() parameters to point the input DEM raster and reference the watershed polygon cutline created earlier as clip shapes. Specify the clipped raster output filename and location. Set additional arguments like target resolution and raster format for the exported result.

Execute gdal.Warp(), which clips the DEM to the watershed cutline, restricting cells to areas inside the polygon. This extracts only necessary raster data for watershed analysis. Finally, export the clipped raster to file with optimized resolution, size, and format settings.

Optimizing Clipping Performance

Productive workflows require balancing processing time against output quality. The following techniques help optimize performance when clipping large rasters with Python and GDAL:

Parallel processing – The gdal.Warp() function supports multi-threaded processing to leverage multiple CPU cores simultaneously, accelerating execution.
Compression – Formats like JPEG2000 apply compression that reduces exported raster file sizes while retaining information quality.
Tiled storage – Tiling rasters helps applications access necessary regions without loading entire files.
Precision – Cell data types like float32 require less storage than float64 while meeting analysis needs.

Test different combinations of parameters when clipping numerous large rasters over cycles of editing and rerunning scripts. Assess speed, output quality and storage considerations for your computational environment and analysis objectives. Automating this experimentation process with Python enables both productivity gains and optimization.

Example Codes

These examples demonstrate Python scripts for clipping rasters in QGIS using gdal.Warp() and vector cutlines. They apply the concepts covered in previous sections. Refer to comments for explanations and customization guidance.

Example 1: Clipping raster to watershed polygon

“`python
# Import modules
from osgeo import gdal
import ogr

# Load vector watershed layer
watershed_path = ‘/watershed.shp’
watershed_layer = iface.addVectorLayer(watershed_path, ”, ‘ogr’)

# Create single watershed polygon geometry as cutline
watershed_poly = watershed_layer.getFeature(0).geometry()
watershed_cutline = watershed_poly.convertToType(QgsWkbTypes.PolygonGeometry)

# Input and output raster paths
raster_in = ‘/DEM.tif’
raster_out = ‘/watershed_DEM.tif’

# Clip raster using polygon cutline
gdal.Warp(raster_out, raster_in, cutlineDSName=watershed_cutline,
cropToCutline=True)

print(‘Clipping complete!’)
“`

Example 2: Batch clipping rasters to grid vector

“`python
# Import modules
import glob, gdal, ogr

# Get list of raster paths with glob
raster_paths = glob.glob(‘/raster_data/*.tif’)

# Load vector grid layer
grid_path = ‘/grid_cells.shp’
grid_layer = iface.addVectorLayer(grid_path, ”, ‘ogr’)

# Iterate rasters clipping each to grid
for raster_path in raster_paths:

filename = os.path.basename(raster_path)
raster_out = f’/output/clips/{filename}’

# Get first grid feature geometry as cutline
grid_poly = grid_layer.getFeature(0).geometry()
cutline = grid_poly.convertToType(QgsWkbTypes.PolygonGeometry)

# Clip raster using grid polygon
gdal.Warp(raster_out, raster_path, cutlineDSName=cutline,
cropToCutline=True)

print(f’Finished clipping {len(raster_paths)} rasters!’)
“`

Conclusion and Further Reading

Python scripting enables automation of raster clipping using GDAL through QGIS. Workflows process large datasets, extract AOIs, and export files with custom parameters. Vector polygons provide flexible cutlines to clip input rasters against.

Further examples demonstrating geospatial analysis workflows using Python and GDAL are available from the following resources:

QGIS Documentation – PyQGIS Developer Cookbook
Geospatial Python – Online Book
Automating GIS Processes – Textbook

Key concepts covered here form a foundation for building more advanced geospatial data workflows. Python programming skills combined with GIS tools like GDAL and QGIS provide abundant opportunities for customization.