Creating Monthly Mean Composite of LAI using Modis Dataset

Question

I am trying to calculate the monthly LAI composite, but somehow code is not working. Need Suggestions? It shows this error, Line 36: image.get is not a function.

// Define the start and end dates
var startDate = '2006-01-01';
var endDate = '2020-12-31';

// Function to calculate monthly LAI mean
var calculateMonthlyLAI = function(year, month) {
  var startDate = ee.Date.fromYMD(year, month, 1);
  var endDate = startDate.advance(1, 'month');
  
  var lai = ee.ImageCollection('MODIS/061/MOD15A2H')
              .filterBounds(region)
              .filterDate(startDate, endDate)
              .select('Lai_500m')
              .mean()
              .set('system:time_start', startDate.millis())
              .set('system:time_end', endDate.millis());
  
  return lai;
};

// Create an empty image collection to store the monthly LAI
var monthlyLAI = ee.ImageCollection([]);

// Iterate over the years and months
for (var year = 2006; year <= 2020; year++) {
  for (var month = 1; month <= 12; month++) {
    var lai = calculateMonthlyLAI(year, month);
    var temporaryCollection = ee.ImageCollection([lai]); // Create a temporary collection with the single lai image
    monthlyLAI = monthlyLAI.merge(temporaryCollection);
  }
}

// Function to export an image to Google Drive
var exportImage = function(image) {
  var dateString = ee.Date(image.get('system:time_start')).format('YYYY-MM');
  var fileName = ee.String('monthly_lai_').cat(dateString);
  
  Export.image.toDrive({
    image: ee.Image(image),
    description: fileName,
    folder: 'GEE_exports',
    scale: 500,
    region: region,
    fileFormat: 'GeoTIFF'
  });
};

// Export each monthly LAI image
monthlyLAI.toList(monthlyLAI.size()).getInfo().forEach(function(image) {
  exportImage(image);
});

Answer 1

When calling getInfo(), you're turning your images into client-side objects. The will not have EE methods like get().

The trick I use when I need to export a set of images is to create a list of unique properties, using ee.ImageCollection.aggregate_array(). Then I turn this into a client-side list with evaluate() (try to avoid getInfo(), as it freezes the GUI). I do client-side iteration on this list, using each element to filter down the collection to the corresponding image. This image is then finally exported.

There is a second issue with your script. The fileName is a server-side object, but the description in toDrive() must be client-side. In your case, you could use the fileName as your unique property. That way you'll get it converted to a client-side string.

var calculateMonthlyLAI = function(year, month) {
  var startDate = ee.Date.fromYMD(year, month, 1);
  var endDate = startDate.advance(1, 'month');
  var lai = ee.ImageCollection('MODIS/061/MOD15A2H')
    .filterBounds(region)
    .filterDate(startDate, endDate)
    .select('Lai_500m')
    .mean()
    .set('system:time_start', startDate.millis())
    .set('system:time_end', endDate.millis())
    .set('fileName', ee.String('monthly_lai_').cat(startDate.format('YYYY-MM')))
  return lai;
};

...

var exportImage = function(image, fileName) {
  Export.image.toDrive({
    image: image,
    description: fileName,
    folder: 'GEE_exports',
    scale: 500,
    region: region,
    fileFormat: 'GeoTIFF'
  });
};
monthlyLAI.aggregate_array('fileName').evaluate(function (fileNames) {
  fileNames.forEach(function(fileName) {
    var image = monthlyLAI
      .filter(ee.Filter.eq('fileName', fileName))
      .first()
    exportImage(image, fileName);
  })
})

https://code.earthengine.google.com/66eccdffb29344d3b1014e0b62a85cee