08_gradients.ipynb

Kernel: Python 3

View source on GitHub

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Run in Google Colab

Gradients

You can compute the gradient of each band of an image with image.gradient(). For example, the following code computes the gradient magnitude and direction of the Landsat 8 panchromatic band:

Install Earth Engine API and geemap

Install the Earth Engine Python API and geemap. The geemap Python package is built upon the ipyleaflet and folium packages and implements several methods for interacting with Earth Engine data layers, such as Map.addLayer(), Map.setCenter(), and Map.centerObject(). The following script checks if the geemap package has been installed. If not, it will install geemap, which automatically installs its dependencies, including earthengine-api, folium, and ipyleaflet.

Important note: A key difference between folium and ipyleaflet is that ipyleaflet is built upon ipywidgets and allows bidirectional communication between the front-end and the backend enabling the use of the map to capture user input, while folium is meant for displaying static data only (source). Note that Google Colab currently does not support ipyleaflet (source). Therefore, if you are using geemap with Google Colab, you should use import geemap.foliumap. If you are using geemap with binder or a local Jupyter notebook server, you can use import geemap, which provides more functionalities for capturing user input (e.g., mouse-clicking and moving).

# Installs geemap package
import subprocess

try:
    import geemap
except ImportError:
    print("geemap package not installed. Installing ...")
    subprocess.check_call(["python", "-m", "pip", "install", "geemap"])

# Checks whether this notebook is running on Google Colab
try:
    import google.colab
    import geemap.foliumap as emap
except:
    import geemap as emap

# Authenticates and initializes Earth Engine
import ee

try:
    ee.Initialize()
except Exception as e:
    ee.Authenticate()
    ee.Initialize()

Create an interactive map

The default basemap is Google Satellite. Additional basemaps can be added using the Map.add_basemap() function.

Map = geemap.Map(center=[40, -100], zoom=4)
Map.add_basemap("ROADMAP")  # Add Google Map
Map

Add Earth Engine Python script

# Load a Landsat 8 image and select the panchromatic band.
image = ee.Image("LANDSAT/LC08/C01/T1/LC08_044034_20140318").select("B8")

# Compute the image gradient in the X and Y directions.
xyGrad = image.gradient()

# Compute the magnitude of the gradient.
gradient = xyGrad.select("x").pow(2).add(xyGrad.select("y").pow(2)).sqrt()

# Compute the direction of the gradient.
direction = xyGrad.select("y").atan2(xyGrad.select("x"))

# Display the results.
Map.setCenter(-122.054, 37.7295, 10)
Map.addLayer(direction, {"min": -2, "max": 2}, "direction")
Map.addLayer(gradient, {"min": -7, "max": 7}, "gradient")

Map.addLayerControl()
Map

Note that gradient() outputs two bands: the gradient in the X-direction and the gradient in the Y-direction. As shown in the example, the two directions can be combined to get gradient magnitude and direction. The magnitude should look something like Figure 1.