Google Earth Engine – Calculate Monthly Average NDVI and Format Date Correctly

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I'm trying to create a chart that is a monthly average for each year from a 22-year MODIS NDVI image collection. I have a number of steps to complete. This is one of them. The closest answer I could find is from the solution in this post:

Google Earth Engine: calculating and plotting monthly average NDVI for a region from MODIS data

The code at the end of the above link uses xProperty: 'system:index' giving the x axis as numbers from 0 onwards. How can I use the year (y) and month (m to create a xProperty that is formatted as a date?

 //Define chart parameters
var chartParam = {
 title: 'Monthly average NDVI',
  hAxis: {title: 'Time'},
  vAxis: {title: 'Average NDVI'},
  };

//Plot the chart
var chart = ui.Chart.image.seriesByRegion({
  imageCollection: yrMo,
  regions: ukr,
  reducer: ee.Reducer.mean(),
  scale: 500,
   xProperty: 'system:index',
 seriesProperty: 'PROJECT'
    });
    //Print chart to console
    print(chart.setOptions(chartParam));

Best Answer

Instead of setting month and year in the mapped function, set a combined date parameter, and specify that as the x-axis:

var yrMo = ee.ImageCollection.fromImages(
  years.map(function (y) {
        return months.map(function (m) {
            return collection
              .filter(ee.Filter.calendarRange(y, y, 'year'))
              .filter(ee.Filter.calendarRange(m, m, 'month'))
              .mean()
              .set('date',ee.Date.fromYMD(y, m, 1).format("YYYY/MM"))

        });
    }).flatten());
...
//Plot the chart
var chart = ui.Chart.image.seriesByRegion({
  imageCollection: yrMo,
  regions: ukr,
  reducer: ee.Reducer.mean(),
  scale: 500,
  xProperty: 'date',
  seriesProperty: 'PROJECT'
});

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Google Earth Engine – Extract SST by Month Per Year Using JavaScript

var sst = ee.ImageCollection('NASA/OCEANDATA/MODIS-Aqua/L3SMI')
    .select('sst')
    .filterDate(ee.Date('2013-01-01'), ee.Date('2017-12-31'))

var months = ee.List.sequence(1, 12);
var years = ee.List.sequence(2013, 2017);

var byMonthYear = ee.ImageCollection.fromImages(
  years.map(function(y) {
    return months.map(function (m) {
      return sst
        .filter(ee.Filter.calendarRange(y, y, 'year'))
        .filter(ee.Filter.calendarRange(m, m, 'month'))
        .mean()
        .set('month', m).set('year', y);
  });
}).flatten());
print(byMonthYear)

Google Earth Engine – Exporting Max NDVI per Month & Year to CSV and Fixing -999 Values

You can obtain the answer you want with the small geometry you use using a different approach. Note that a Landsat pixel and the scale you are currently using is 30m. Your geometry is just about 10x10m.

Add the moment you are using an unweighted reducer (ee.Reducer.max), which will only take pixels into account if the centroid is within the geometry (see here). Neither the geometry nor the scale you are using apparently include the centroid of the pixels, thus you return no valid values (which you set to -999 afterwards).

You have two options:

1) Use a scale which will include the centroid and use the unweighted reducer. Set the scale for example at 10m or calculate approximately the scale of a square geometry using:

var scale = table.geometry().area(1).sqrt();

2) Use a weighted reducer which will include pixels which approximately include at least 0.5% of the pixel. An example would be using a mean reducer. In such a small region you probably aggregate one pixel, so that is fine for your max outcome.

var data = cloudlessNDVI.filterDate(ini,end).max().reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: feature.geometry(),
  scale: 30
});

Here an example returning both values (and you will see they are similar, confirming there is just one pixel in the region).

Best Answer

Related Solutions

Google Earth Engine – Extract SST by Month Per Year Using JavaScript

Google Earth Engine – Exporting Max NDVI per Month & Year to CSV and Fixing -999 Values

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