Crop raster data with rioxarray and geopandas

Estimated time: 100 minutes

Overview

Questions

• How can I crop my raster data to the area of interest?

Objectives

• Crop raster data with a bounding box.
• Crop raster data with a polygon.
• Match two raster datasets in different CRS.

It is quite common that the raster data you have in hand is too large to process, or not all the pixels are relevant to your area of interest (AoI). In both situations, you should consider cropping your raster data before performing data analysis.

In this episode, we will introduce how to crop raster data into the desired area. We will use one Sentinel-2 image over Amsterdam as the example raster data, and introduce how to crop your data to different types of AoIs.

Introduce the Data

We will use the results of the satellite image search: search.json, which is generated in an exercise from Episode 5: Access satellite imagery using Python.

If you would like to work with the data for this lesson without downloading data on-the-fly, you can download the raster data using this link. Save the geospatial-python-raster-dataset.tar.gz file in your current working directory, and extract the archive file by double-clicking on it or by running the following command in your terminal tar -zxvf geospatial-python-raster-dataset.tar.gz. Use the file geospatial-python-raster-dataset/search.json (instead of search.json) to get started with this lesson.

We also use the cropped fields polygons fields_cropped.shp, which was generated in an exercise from Episode 7: Vector data in python.

Align the CRS of the raster and the vector data

---

We load a true color image using pystac and rioxarray and check the shape of the raster:

PYTHON

import pystac
import rioxarray

# Load image and inspect the shape
items = pystac.ItemCollection.from_file("search.json")
raster = rioxarray.open_rasterio(items[1].assets["visual"].href) # Select a true color image
print(raster.shape)

OUTPUT

(3, 10980, 10980)

This will perform a “lazy” loading of the image, i.e. the image will not be loaded into the memory until necessary, but we can still access some attributes, e.g. the shape of the image.

The large size of the raster data makes it time and memory consuming to visualize in its entirety. Instead, we can fetch and plot the overviews of the raster. “Overviews” are precomputed lower resolution representations of a raster, stored in the same COG that contains the original raster.

PYTHON

# Get the overview asset
raster_overview = rioxarray.open_rasterio(items[1].assets["visual"].href, overview_level=3)
print(raster_overview.shape)

# Visualize it
raster_overview.plot.imshow(figsize=(8,8))

As we can see, the overview image is much smaller compared to the original true color image.

To align the raster and vector data, we first check each coordinate system. For raster data, we use pyproj.CRS:

PYTHON

from pyproj import CRS

# Check the coordinate system
CRS(raster.rio.crs)

OUTPUT

<Derived Projected CRS: EPSG:32631>
Name: WGS 84 / UTM zone 31N
Axis Info [cartesian]:
- [east]: Easting (metre)
- [north]: Northing (metre)
Area of Use:
- undefined
Coordinate Operation:
- name: UTM zone 31N
- method: Transverse Mercator
Datum: World Geodetic System 1984
- Ellipsoid: WGS 84
- Prime Meridian: Greenwich

To open and check the coordinate system of vector data, we use geopandas:

PYTHON

import geopandas as gpd

# Load the polygons of the crop fields
fields = gpd.read_file("fields_cropped.shp")

# Check the coordinate system
fields.crs

OUTPUT

<Derived Projected CRS: EPSG:28992>
Name: Amersfoort / RD New
Axis Info [cartesian]:
- X[east]: Easting (metre)
- Y[north]: Northing (metre)
Area of Use:
- name: Netherlands - onshore, including Waddenzee, Dutch Wadden Islands and 12-mile offshore coastal zone.
- bounds: (3.2, 50.75, 7.22, 53.7)
Coordinate Operation:
- name: RD New
- method: Oblique Stereographic
Datum: Amersfoort
- Ellipsoid: Bessel 1841
- Prime Meridian: Greenwich

As seen, the coordinate systems differ. To crop the raster using the shapefile, we first need to reproject one dataset to the other’s CRS. Since raster is large, we will convert the CRS of fields to the CRS of raster to avoid loading the entire image:

PYTHON

fields = fields.to_crs(raster.rio.crs)

Crop raster data with a bounding box

---

The clip_box function allows one to crop a raster by the min/max of the x and y coordinates. Note that we are cropping the original image raster now, and not the overview image raster_overview.

PYTHON

# Crop the raster with the bounding box
raster_clip_box = raster.rio.clip_box(*fields.total_bounds)
print(raster_clip_box.shape)

OUTPUT

(3, 1565, 1565)

We successfully cropped the raster to a much smaller piece. We can visualize it now:

PYTHON

raster_clip_box.plot.imshow(figsize=(8,8))

This cropped image can be saved for later usage:

PYTHON

raster_clip_box.rio.to_raster("raster_clip.tif")

Crop raster data with polygons

---

We have a cropped image around the fields. To further analyze the fields, one may want to crop the image to the exact field boundaries. This can be done with the clip function:

PYTHON

raster_clip_fields = raster_clip_box.rio.clip(fields['geometry'])

And we can visualize the results:

PYTHON

raster_clip_fields.plot.imshow(figsize=(8,8))

Exercise: crop raster data with a specific code

In the column “gewascode” (translated as “crop code”) of fields, you can find the code representing the types of plants grown in each field. Can you:

1. Select the fields with “gewascode” equal to 257;
2. Crop the raster raster_clip_box with the selected fields;
3. Visualize the cropped image.

Show me the solution

PYTHON

mask = fields['gewascode']==257
fields_gwascode = fields.where(mask)
fields_gwascode = fields_gwascode.dropna()
raster_clip_fields_gwascode = raster_clip_box.rio.clip(fields_gwascode['geometry'])
raster_clip_fields_gwascode.plot.imshow(figsize=(8,8))

Crop raster data using reproject_match() function

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So far we have learned how to crop raster images with vector data. We can also crop a raster with another raster data. In this section, we will demonstrate how to crop the raster_clip_box image using the raster_clip_fields_gwascode image. We will use the reproject_match function. As indicated by its name, it performs reprojection and clipping in one go.

To demonstrate the reprojection, we will first reproject raster_clip_fields_gwascode to the RD CRS system, so it will be in a different CRS from raster_clip_box:

PYTHON

# Reproject to RD to make the CRS different from the "raster"
raster_clip_fields_gwascode = raster_clip_fields_gwascode.rio.reproject("EPSG:28992")
CRS(raster_clip_fields_gwascode.rio.crs)

OUTPUT

<Derived Projected CRS: EPSG:28992>
Name: Amersfoort / RD New
Axis Info [cartesian]:
- [east]: Easting (metre)
- [north]: Northing (metre)
Area of Use:
- undefined
Coordinate Operation:
- name: unnamed
- method: Oblique Stereographic
Datum: Amersfoort
- Ellipsoid: Bessel 1841
- Prime Meridian: Greenwich

And let’s check again the CRS of raster_clip_box:

PYTHON

CRS(raster_clip_box.rio.crs)

OUTPUT

<Derived Projected CRS: EPSG:32631>
Name: WGS 84 / UTM zone 31N
Axis Info [cartesian]:
- [east]: Easting (metre)
- [north]: Northing (metre)
Area of Use:
- undefined
Coordinate Operation:
- name: UTM zone 31N
- method: Transverse Mercator
Datum: World Geodetic System 1984
- Ellipsoid: WGS 84
- Prime Meridian: Greenwich

Now the two images are in different coordinate systems. We can use rioxarray.reproject_match() function to crop raster_clip_box image.

PYTHON

raster_reproject_match = raster_clip_box.rio.reproject_match(raster_clip_fields_gwascode)
raster_reproject_match.plot.imshow(figsize=(8,8))

We can also use it to expand raster_clip_fields_gwascode to the extent of raster_clip_box:

PYTHON

raster_reproject_match = raster_clip_fields_gwascode.rio.reproject_match(raster_clip_box)
raster_reproject_match.plot.imshow(figsize=(8,8))

In one line reproject_match does a lot of helpful things:

1. It reprojects.
2. It matches the extent using nodata values or by clipping the data.
3. It sets nodata values. This means we can run calculations on those two images.

Code Tip

As we saw before, there also exists a method called reproject(), which only reprojects one raster to another projection. If you want more control over how rasters are resampled, clipped, and/or reprojected, you can use the reproject() method and other rioxarray methods individually.

Key Points

• Use clip_box to crop a raster with a bounding box.
• Use clip to crop a raster with a given polygon.
• Use reproject_match to match two raster datasets.