I am trying to calculate the area of each class of my grid. My code reference is from here.

My code: Earth engine link

var grid = ee.FeatureCollection("users/miaohu828/finalGridexpot1000")
var modisLandcover = ee.ImageCollection("MODIS/006/MCD12Q1")
var bp = ee.FeatureCollection("users/miaohu828/bankment_polygon")

var filtered2018 = modisLandcover.filter(
  ee.Filter.date('2018-01-01', '2018-12-31'))
// The filtered collection has 1 classified image for 2018
// We can call .first() to get that element and cast it to an image
var landcover2018 = ee.Image(filtered2018.first())
// Each year's image contains multiple bands with different classification scheme
// We pick the band 'LC_Type1' for this example and extract it.
var classified2018 = landcover2018.select('LC_Type1')
// There are 17 classes in the scheme as described in https://developers.google.com/earth-engine/datasets/catalog/MODIS_006_MCD12Q1
// We pick the color associated with each class from the documentation and create a palette
var palette = ['05450a', '086a10', '54a708', '78d203', '009900',//forest
               'c6b044', 'dcd159','fbff13', //shrub, savannah
               'b6ff05', //grassland
               '27ff87', //wetland
               '69fff8', //snow
               'f9ffa4', //Barren
var vis_params = {min: 0, max: 17, palette: palette};
// Visualize the classified image
//Map.addLayer(classified, {min:1, max:17, palette: palette}, 'MODIS Landcover 2018')
var bpLandcover2018 = classified2018.clip(bp)
Map.addLayer(bpLandcover2018,{min:1, max:17, palette: palette},'Yangtze Land Cover 2018')

// Add a new property 'grid_name'
var finalGrid = grid.map(function(f) {
  return f.set('grid_name', ee.String('grid_').cat(f.get('system:index')))

var calculateClassArea = function(feature) {
    var areas = ee.Image.pixelArea().addBands(classified2018).reduceRegion({
      reducer: ee.Reducer.sum().group({
      groupField: 1,
      groupName: 'class',
    geometry: feature.geometry(),
    scale: 500,
    maxPixels: 1e10

    var classAreas = ee.List(areas.get('groups'))
    var classAreaLists = classAreas.map(function(item) {
      var areaDict = ee.Dictionary(item)
      var classNumber = ee.Number(
      var area = ee.Number(
      return ee.List([classNumber, area])
    var result = ee.Dictionary(classAreaLists.flatten())
    var grid_name = feature.get('grid_name')
    return ee.Feature(
      result.set('grid_name', grid_name))

var districtAreas = grid.map(calculateClassArea)
var classes = ee.List.sequence(1, 17)
var outputFields = ee.List(['grid_name']).cat(classes).getInfo()

    collection: districtAreas,
    description: 'class_area_by_grid',
    folder: 'earthengine',
    fileNamePrefix: 'class_area_by_grid',
    fileFormat: 'CSV',
    selectors: outputFields

My code runs with an error:

FeatureCollection (Error)
Error in map(ID=000000000000000000ab):
Dictionary. set: Parameter 'value' is required.

I can't find a way to solve it.

  • 5
    Change 'grid' into 'grid_name' in line 56. var grid_name = feature.get('grid_name') May 13, 2021 at 5:02

1 Answer 1


I think the problem is here:

var grid_name = feature.get('grid_name')

Try to have the same name with your feature.

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