Adding columns to a FeatureCollection

Question

I'm trying to build a FeatureCollection from 4 differents FeatureCollection in order to export an excel with 4 columns containg the 4 differents lists. Here is an example of the strucuture of each FeatureCollection And how I get them :

var chloaFeatures = ee.FeatureCollection(ee.List(chloa_4kmOnly
  .reduceRegion({
    reducer : ee.Reducer.toList(),
    geometry : geometry, 
    crs : ModisProjection
  })
  .get('Chloa_OC3'/*,'SR_B1','SR_B2','SR_B3'*/))
  .map(function(chloa){
    return ee.Feature(null, {chloa_4kmOnly : chloa})
  })
)

var B1Features = ee.FeatureCollection(ee.List(chloa_4kmOnly
  .reduceRegion({
    reducer : ee.Reducer.toList(),
    geometry : geometry, 
    crs : ModisProjection
  })
  .get('SR_B1'))
  .map(function(chloa){
    return ee.Feature(null, {B1 : chloa})
  })
)

var B2Features = ee.FeatureCollection(ee.List(chloa_4kmOnly
  .reduceRegion({
    reducer : ee.Reducer.toList(),
    geometry : geometry, 
    crs : ModisProjection
  })
  .get('SR_B2'))
  .map(function(chloa){
    return ee.Feature(null, {B2 : chloa})
  })
)

var B3Features = ee.FeatureCollection(ee.List(chloa_4kmOnly
  .reduceRegion({
    reducer : ee.Reducer.toList(),
    geometry : geometry, 
    crs : ModisProjection
  })
  .get('SR_B3'))
  .map(function(chloa){
    return ee.Feature(null, {B3 : chloa})
  })
)

You can find my complete code here: https://code.earthengine.google.com/4ceaa4d6f96e1107f2f53af572e3badd

Answer 1

It is easier (and a lot more efficient) to do a single reduceRegion() and transform the resulting dictionary into your feature collection. That transformation requires a bit of fiddling, but here's a reasonably easy way to do it:

var valuesByBand = chloa_4kmOnly
  .reduceRegion({
    reducer : ee.Reducer.toList(),
    geometry : geometry, 
    crs : ModisProjection
  })

var features = ee.FeatureCollection(
  ee.Array(valuesByBand.values())
    .transpose()
    .toList()
    .map(function (values) {
      return ee.Feature(null, ee.Dictionary.fromLists(
        valuesByBand.keys(), 
        values
      ))
    })
)
Export.table.toDrive({
  collection: features,
  description: 'Features_Landsat',
  folder: 'tanganyika',
  fileFormat: 'CSV',
  selectors: ["Chloa_OC3", "SR_B1", "SR_B2", "SR_B3"]
})

https://code.earthengine.google.com/014c1b5812232b8399d12485c83fdcd2