Trying to export a feature collection in Google Earth Engine

Question

I have obtained sea surface temperature data for Ascension Island and now want to export this into a csv. I managed to export the mean SST values for a previous package I was looking at (NASA/OCEANDATA/MODIS-Aqua/L3SMI) but I am now trying to use the exact same code using a different package (JAXA/GCOM-C/L3/OCEAN/SST/V3) and it won't provide me with the SST values, it just gives me these values instead:

system:index      date                .geo

20230101A   2023-01-01  {"type":"MultiPoint","coordinates":[]}

20230101D   2023-01-01  {"type":"MultiPoint","coordinates":[]}

20230102A   2023-01-02  {"type":"MultiPoint","coordinates":[]}.

This is the code for the package that did work:

// 5.5 - Time series and marine data

// Define the region of interest (ROI) around Ascension Island
var ROI = ee.FeatureCollection([
  ee.Feature(
    ee.Geometry.Rectangle([-14.57, -8.12, -14.17, -7.75]), {label: 'Ascension Island'})
]);

Map.setCenter(-14.5, -8.1, 8);
Map.addLayer(ROI, {}, 'ROI');

// Adding names to ROI
var FeatCollectAsAList = ROI.toList(ROI.size());
print(FeatCollectAsAList, 'ROI');


// Map a function over the list to rename the IDs
var ROIEdit = ee.FeatureCollection(FeatCollectAsAList.map(function(feat){
  var id = ee.Feature(feat).get('id');
  return ee.Feature(feat).set("system:index", id);
}));

print(ROIEdit);

// Obtain SST data from NASA MODIS
var AI_SST = ee.ImageCollection('NASA/OCEANDATA/MODIS-Aqua/L3SMI').select('sst') // use OISST for EEZ_SE
    .filterDate(ee.Date('2017-01-01'), ee.Date('2018-04-01'))
    .filterBounds(ROI);


// Convert ImageCollection to FeatureCollection for export
var exportFeatures = AI_SST.map(function(image) {
  var date = image.date().format('YYYY-MM-dd');
  var mean = image.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: ROI.geometry(),
    scale: 500,
    maxPixels: 1e9
  });

  return ee.Feature(null, mean.set('date', date));
});

// Print a sample feature to check if data is being processed correctly
print('Sample feature:', exportFeatures.first());

// Export the data to Google Drive as CSV
Export.table.toDrive({

  collection: exportFeatures,
  description: 'SST_Ascension_Island_2017_2018',
  fileFormat: 'CSV',
  folder: 'EarthEngineExports' // Optional: specify a folder in Google Drive
});

This is the code for the package that did NOT work:

// 5.5 - Time series and marine data

// Define the region of interest (ROI) around Ascension Island
var ROI = ee.FeatureCollection([
  ee.Feature(
    ee.Geometry.Rectangle([-14.57, -8.12, -14.17, -7.75]), {label: 'Ascension Island'})
]);

Map.setCenter(-14.5, -8.1, 8);
Map.addLayer(ROI, {}, 'ROI');

// Adding names to ROI
var FeatCollectAsAList = ROI.toList(ROI.size());
print(FeatCollectAsAList, 'ROI');

// Map a function over the list to rename the IDs
var ROIEdit = ee.FeatureCollection(FeatCollectAsAList.map(function(feat){
  var id = ee.Feature(feat).get('id');
  return ee.Feature(feat).set("system:index", id);
}));

print(ROIEdit);

// Obtain SST data from NASA MODIS
var AI_SST = ee.ImageCollection("JAXA/GCOM-C/L3/OCEAN/SST/V3").select('SST_AVE')
    .filterDate(ee.Date('2023-01-01'), ee.Date('2024-04-01'))
    .filterBounds(ROI);

// Convert ImageCollection to FeatureCollection for export
var exportFeatures = AI_SST.map(function(image) {
  var date = image.date().format('YYYY-MM-dd');
  var mean = image.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: ROI.geometry(),
    scale: 500,
    maxPixels: 1e9
  });
  
  
  return ee.Feature(null, mean.set('date', date));
});

print('Sample feature:', exportFeatures.first());

// Export the data to Google Drive as CSV
Export.table.toDrive({
  collection: exportFeatures,
  description: 'SST_Ascension_Island_2023_2024_2',
  fileFormat: 'CSV',
  folder: 'EarthEngineExports' 
});

Does anyone know why the exported data is different for the different packages? I have managed to obtain the SST values I wanted by downloading the data from a chart I made, but I am just curious to know why it hasn't worked so that hopefully I know how to solve this next time I come across this problem.

Answer 1

I think the issue is that the first feature in the exportFeatures has no value for the SST_AVE attribute. This is because the first image in the JAXA/GCOM-C/L3/OCEAN/SST/V3 collection contains only masked values over the ROI. Thus, my guess is that by default, Export.table.toDrive does not export the featureCollection with the SST_AVE attribute. A workaround is to define the attributes you wish to export using selectors.

// Export the data to Google Drive as CSV
Export.table.toDrive({
  collection: exportFeatures,
  selectors: ['SST_AVE', 'date'],
  description: 'SST_Ascension_Island_2023_2024_2',
  fileFormat: 'CSV',
  folder: 'EarthEngineExports' 
});