Join Overview

Joins are used to combine elements from different collections (e.g. ImageCollection or FeatureCollection) based on a condition specified by an ee.Filter. The filter is constructed with arguments for the properties in each collection that are related to each other. Specifically, leftField specifies the property in the primary collection that is related to the rightField in the secondary collection. The type of filter (e.g. equals, greaterThanOrEquals, lessThan, etc.) indicates the relationship between the fields. The type of join indicates one-to-many or one-to-one relationships between the elements in the collections and how many matches to retain. The output of a join is produced by join.apply() and will vary according to the type of join.

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Google Earth Engine API