[GIS] Exporting time series of multiple polygons to .csv

google-earth-engine

I defined (by drawing) a significant number of polygons and need to export the time series of the average values inside each one. Following this example, that allows to display charts for each region, how can I export the data to a .csv? Ideally I will have in the first row the names of the regions.

// Define a FeatureCollection: regions of the American West.
var regions = ee.FeatureCollection([
  ee.Feature(    // San Francisco.
    ee.Geometry.Rectangle(-122.45, 37.74, -122.4, 37.8), {label: 'City'}),
  ee.Feature(  // Tahoe National Forest.
    ee.Geometry.Rectangle(-121, 39.4, -120.8, 39.8), {label: 'Forest'}),
  ee.Feature(  // Black Rock Desert.
    ee.Geometry.Rectangle(-119.15, 40.8, -119, 41), {label: 'Desert'})
]);

// Load Landsat 8 brightness temperature data for 1 year.
var temps2013 = ee.ImageCollection('LANDSAT/LC8_L1T_32DAY_TOA')
    .filterDate('2012-12-25', '2013-12-25')
    .select('B11');

// Create a time series chart.
var tempTimeSeries = ui.Chart.image.seriesByRegion(
    temps2013, regions, ee.Reducer.mean(), 'B11', 200, 'system:time_start', 'label')
        .setChartType('ScatterChart')
        .setOptions({
          title: 'Temperature over time in regions of the American West',
          vAxis: {title: 'Temperature (Kelvin)'},
          lineWidth: 1,
          pointSize: 4,
          series: {
            0: {color: 'FF0000'}, // urban
            1: {color: '00FF00'}, // forest
            2: {color: '0000FF'}  // desert
}});

// Display.
print(tempTimeSeries);

Best Answer

After printing the chart to the console tab:

In the console tab, click on the "enlarge icon" to display the chart in a full window.
In the full window, click on "Download CSV"

The first row of the resulting CSV file includes the names of the regions.

system:time_start,City,Forest,Desert
"Apr 7, 2013",297.119,273.622,286.196
"May 9, 2013",300.654,299.472,275.405
"Jun 10, 2013",287.004,300.091,320.261
"Jul 12, 2013",286.198,298.332,315.787
"Aug 13, 2013",307.255,295.935,311.532
"Sep 14, 2013",291.677,276.471,294.359
"Oct 16, 2013",285.385,283.333,279.032
"Nov 17, 2013",282.925,273.038,264.722
"Dec 19, 2013",286.648,277.89,256.074

Related Solutions

csv – Exporting Time Series of Multiple Polygons to CSV Format Table

You can remove the geometry by using .setGeometry(null) on a feature.

To remove every geometry you need to map over the collection. In your script that could be in the same function as setting the imageID

return f.set('imageId', image.id()).setGeometry(null);

As a full script:

var regions = ee.FeatureCollection([
  ee.Feature(    // San Francisco.
    ee.Geometry.Rectangle(-122.45, 37.74, -122.4, 37.8), {label: 'City'}),
  ee.Feature(  // Tahoe National Forest.
    ee.Geometry.Rectangle(-121, 39.4, -120.8, 39.8), {label: 'Forest'}),
  ee.Feature(  // Black Rock Desert.
    ee.Geometry.Rectangle(-119.15, 40.8, -119, 41), {label: 'Desert'})
]);

var temps2013 = ee.ImageCollection('LANDSAT/LC8_L1T_32DAY_TOA')
    .filterDate('2012-12-25', '2013-12-25')
    .select('B11');
var triplets = temps2013.map(function(image) {
  return image.select('B11').reduceRegions({
    collection: regions.select(['label']), 
    reducer: ee.Reducer.mean(), 
    scale: 30
  }).filter(ee.Filter.neq('mean', null))
    .map(function(f) { 
      return f.set('imageId', image.id()).setGeometry(null);
    });
}).flatten();
print(triplets.first());
var format = function(table, rowId, colId) {
  var rows = table.distinct(rowId);

  var joined = ee.Join.saveAll('matches').apply({
    primary: rows, 
    secondary: table, 
    condition: ee.Filter.equals({
      leftField: rowId, 
      rightField: rowId
    })
  });

  return joined.map(function(row) {
      var values = ee.List(row.get('matches'))

        .map(function(feature) {
          feature = ee.Feature(feature);
          return [feature.get(colId), feature.get('mean')];
        });
      return row.select([rowId]).set(ee.Dictionary(values.flatten()));
    });
};

var table1 = format(triplets, 'imageId', 'label');

var desc1 = 'table_demo_'; 
Export.table.toDrive({
  collection: table1, 
  description: desc1, 
  fileNamePrefix: desc1,
  fileFormat: 'CSV'
});

Google Earth Engine – How to Create Time Series and Export to CSV

Here is a working script (https://code.earthengine.google.com/ea82c9501fb736f3f615741620f4875a):

// Get  temperature data for 1 year.
var modis = ee.ImageCollection('MODIS/006/MOD11A1');
var modisLST = modis.filterBounds(POI)
                    .filterDate('2003-12-25', '2004-02-25')
                    .select('LST_Day_1km');


// Convert temperature to Celsius.
modisLST = modisLST.map(function(img){
  var date = img.get('system:time_start');
  return img.multiply(0.02).subtract(273.15).set('system_time_start', date);
});

// Create a function that takes an image, calculates the mean over a
// geometry and returns the value and the corresponding date as a 
// feature.
var createTS = function(img){
  var date = img.get('system_time_start');
  var value = img.reduceRegion(ee.Reducer.mean(), POI).get('LST_Day_1km');
  var ft = ee.Feature(null, {'system:time_start': date, 
                             'date': ee.Date(date).format('Y/M/d'), 
                             'value': value});
  return ft;
};

// Apply the function to each image in modisLST.
var TS = modisLST.map(createTS);

// Create a graph of the time-series.
var graph = ui.Chart.feature.byFeature(TS, 'system:time_start', 'value');

print(graph.setChartType("ColumnChart")
           .setOptions({vAxis: {title: 'LST [deg. C]'},
                        hAxis: {title: 'Date'}}));

// Export the time-series as a csv.
Export.table.toDrive({collection: TS, selectors: 'date, value'});

Basically what's happening is that we map over the images in the image-collection, and create a feature for each image (without a geometry). Then we use the feature-collection to export a csv and plot a graph.

Best Answer

Related Solutions

csv – Exporting Time Series of Multiple Polygons to CSV Format Table

Google Earth Engine – How to Create Time Series and Export to CSV

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