Mapping over a FeatureCollection

  • featureCollection.map() applies an operation to each Feature within a FeatureCollection, enabling modifications or calculations on a feature-by-feature basis.

  • This function can be used to add new properties based on geometric calculations or existing property values, illustrated by calculating and adding an area property to each feature.

  • It is also possible to generate an entirely new FeatureCollection by applying a transformation function that creates new features, as demonstrated by converting polygons to their centroids.

  • When creating new features within the map() function, selective property propagation is achievable, allowing control over which attributes are transferred to the new FeatureCollection.

To apply the same operation to every Feature in a FeatureCollection, use featureCollection.map(). For example, to add another area attribute to every feature in a watersheds FeatureCollection, use:

Code Editor (JavaScript)

// Load watersheds from a data table.
var sheds = ee.FeatureCollection('USGS/WBD/2017/HUC06');

// This function computes the feature's geometry area and adds it as a property.
var addArea = function(feature) {
  return feature.set({areaHa: feature.geometry().area().divide(100 * 100)});
};

// Map the area getting function over the FeatureCollection.
var areaAdded = sheds.map(addArea);

// Print the first feature from the collection with the added property.
print('First feature:', areaAdded.first());

Python setup

See the Python Environment page for information on the Python API and using geemap for interactive development.

import ee
import geemap.core as geemap

Colab (Python)

# Load watersheds from a data table.
sheds = ee.FeatureCollection('USGS/WBD/2017/HUC06')

# Map an area calculation function over the FeatureCollection.
area_added = sheds.map(
    lambda feature: feature.set(
        {'areaHa': feature.geometry().area().divide(100 * 100)}
    )
)

# Print the first feature from the collection with the added property.
display('First feature:', area_added.first())

In the previous example, note that a new property is set based on a computation with the feature's geometry. Properties can also be set using a computation involving existing properties.

An entirely new FeatureCollection can be generated with map(). The following example converts the watersheds to centroids:

Code Editor (JavaScript)

// This function creates a new feature from the centroid of the geometry.
var getCentroid = function(feature) {
  // Keep this list of properties.
  var keepProperties = ['name', 'huc6', 'tnmid', 'areasqkm'];
  // Get the centroid of the feature's geometry.
  var centroid = feature.geometry().centroid();
  // Return a new Feature, copying properties from the old Feature.
  return ee.Feature(centroid).copyProperties(feature, keepProperties);
};

// Map the centroid getting function over the features.
var centroids = sheds.map(getCentroid);

// Display the results.
Map.addLayer(centroids, {color: 'FF0000'}, 'centroids');

Python setup

See the Python Environment page for information on the Python API and using geemap for interactive development.

import ee
import geemap.core as geemap

Colab (Python)

# This function creates a new feature from the centroid of the geometry.
def get_centroid(feature):
  # Keep this list of properties.
  keep_properties = ['name', 'huc6', 'tnmid', 'areasqkm']
  # Get the centroid of the feature's geometry.
  centroid = feature.geometry().centroid()
  # Return a new Feature, copying properties from the old Feature.
  return ee.Feature(centroid).copyProperties(feature, keep_properties)

# Map the centroid getting function over the features.
centroids = sheds.map(get_centroid)

# Display the results.
m = geemap.Map()
m.set_center(-96.25, 40, 4)
m.add_layer(centroids, {'color': 'FF0000'}, 'centroids')
m

Note that only a subset of properties is propagated to the features in the new collection.