Extracting NDVI over time using Google Earth Engine

Question

I am trying to extract NDVI data for provinces in Cambodia over time. The following codes work for a specific year (2002).

Can someone show me a better way to collect the data over time automatically on GEE?

var collection = ee.ImageCollection('LANDSAT/LE07/C01/T1_32DAY_NDVI')
.filterDate('2002-01-01', '2002-12-31').select('NDVI');

var mean = collection.reduce(ee.Reducer.mean());

var meanDictionary = mean.reduceRegions({
  collection: Cambodia,
  reducer: ee.Reducer.mean(),
  scale: 30,
});

Export.table.toDrive(meanDictionary,"NDVI","NDVI","NDVI_Cambodia");

Answer 1

You will need to define a list of years you are interested in, and map over that list to get a mean NDVI image for every year. Then use reduceRegion on every image to obtain the mean NDVI value in the geometry. Export the resulting image collection as a table.

// Load collection and geometry
var Cambodia = geometry;
var collection = ee.ImageCollection('LANDSAT/LE07/C01/T1_32DAY_NDVI').select('NDVI');

// Declare years of interest
var years = ee.List.sequence(2000, 2010);

// Map a function to select data within the year and apply mean reducer
var byYear = ee.ImageCollection.fromImages(
    years.map(function(y) {
      var image = collection.filter(ee.Filter.calendarRange(y, y, 'year'))
              .mean();
        // get the mean value for each year
      var meanDictionary = image.reduceRegion({ // use reduce region for single geometry
        geometry: Cambodia,
        reducer: ee.Reducer.mean(),
        scale: 30,
      });
      //set data to image
      return image.set(meanDictionary)
          // add time information to image
          .set('year', y,'system:time_start', ee.Date.fromYMD(y,1,1).millis())
    })
  );

// to test code printing, increase the scale or decrease size goemetry
// print(byYear)

// export the image collection as a table (CSV)
Export.table.toDrive(byYear,"NDVI","NDVI","NDVI_Cambodia");

link code

-- EDIT --

user requested the code for featurecollections:

// Map a function to select data within the year and apply mean reducer
var byYear = ee.FeatureCollection(years.map(function(y) {
      var image = collection.filter(ee.Filter.calendarRange(y, y, 'year'))
              .mean();
        // get the mean value for each year
      var newFC = image.reduceRegions({ 
                              // use reduceRegion for single geometry
                              // use reduceRegions for multiple features
        collection: Cambodia,
        reducer: ee.Reducer.mean(),
        scale: 30,
        tileScale : 1, // increase tileScale if running out of computation or export
      }) // NOTE THAT THIS RETURNS A FEATURE COLLECTION, thus we map over the features to add date
        .map(function(feat){
          return feat.set('year', y,'system:time_start', ee.Date.fromYMD(y,1,1).millis());
        });
        // now we return the featurecollection, not the image
      return newFC; 
    })
  ).flatten(); // flatten() the feature collection of feature collections

link code