Simple Joins

A simple join returns elements from the primary collection that match any element in the secondary collection according to the match condition in the filter. To perform a simple join, use an ee.Join.simple(). This might be useful for finding the common elements among different collections or filtering one collection by another. For example, consider two image collections which (might) have some matching elements, where “matching” is defined by the condition specified in a filter. For example, let matching mean the image IDs are equal. Since the matching images in both collections are the same, use a simple join to discover this set of matching images:

// Load a Landsat 8 image collection at a point of interest.
var collection = ee.ImageCollection('LC8_L1T_TOA')
    .filterBounds(ee.Geometry.Point(-122.09, 37.42));

// Define start and end dates with which to filter the collections.
var april = '2014-04-01';
var may = '2014-05-01';
var june = '2014-06-01';
var july = '2014-07-01';

// The primary collection is Landsat images from April to June.
var primary = collection.filterDate(april, june);

// The secondary collection is Landsat images from May to July.
var secondary = collection.filterDate(may, july);

// Use an equals filter to define how the collections match.
var filter = ee.Filter.equals({
  leftField: 'system:index',
  rightField: 'system:index'
});

// Create the join.
var simpleJoin = ee.Join.simple();

// Apply the join.
var simpleJoined = simpleJoin.apply(primary, secondary, filter);

// Display the result.
print('Simple join: ', simpleJoined);
    

In the previous example, observe that the collections to join temporally overlap by about a month. Note that when this join is applied, the output will be an ImageCollection with only the matching images in the primary collection. The output should look something like:

Image LANDSAT/LC8_L1T_TOA/LC80440342014125LGN00 (12 bands)
Image LANDSAT/LC8_L1T_TOA/LC80440342014141LGN00 (12 bands)
    

This output shows that two images match (as specified in the filter) between the primary and secondary collections, images at day-of-year 125 and 141, or May 5 and May 21.

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