[GIS] Export NDVI from Google Earth Engine

exportgooglegoogle-earth-enginelandsatndvi

I generated a Lansat 5 image NDVI in a rectangle of interest in Google Earth Engine.

After that, I tried to export the image to my Google Drive, but the final result does not load the usual values of an NDVI (-1 to +1), as is usually done in a GIS program (QGIS, ArcGIS, etc.). The DN has a single value (attached image).

How could I solve this?

My code is:

var imgL5 = ee.Image('LANDSAT/LT05/C01/T2_TOA/LT05_226074_19850719');

var poly = ee.Geometry.Polygon([[ 
[-56.298454803020135,-20.398292995561647], 
[-56.298454803020135,-19.8799857811524], 
[-57.037285369426385,-19.8799857811524], 
[-57.037285369426385,-20.398292995561647] 
]])

var cortado = imgL5.clip(poly);

//Calc NDVI

var nir = cortado.select('B5');
var red = cortado.select('B4');
var NDVI = nir.subtract(red).divide(nir.add(red));

Map.addLayer(NDVI);

// Export the image, specifying scale and region.
Export.image.toDrive({
  image: NDVI.float(),
  description: 'NDVI_9',
  scale: 30,
  region: geometry,
  folder: 'GEE',
});


Map.setCenter(-56.67318953949383,-20.121019484901392, 10);

Best Answer

I ran your code. I only updated- "region: poly" (and not- "region: geometry") and it worked fine with Qgis. Maybe the region you defined in the export code was the problem ?

BTW if you want your NDVI values to be between -1 to 1 you can use landsat5 surface reflectance: https://developers.google.com/earth-engine/datasets/catalog/LANDSAT_LT05_C01_T1_SR . In your code you used top of atmosphere data and that's why there is values smaller then -1 .

Related Solutions

[GIS] Writing code for monthly NDVI medians in Google Earth Engine

For the record, here is a good way to do this:

var imageCollection = ee.ImageCollection("LANDSAT/LT05/C01/T1");
var months = ee.List.sequence(1, 12);

var composites = ee.ImageCollection.fromImages(months.map(function(m) {
  var filtered = imageCollection.filter(ee.Filter.calendarRange({
    start: m,
    field: 'month'
  }));
  var composite = ee.Algorithms.Landsat.simpleComposite(filtered);
  return composite.normalizedDifference(['B4', 'B3']).rename('NDVI')
      .set('month', m);
}));
print(composites);

var check = ee.Image(composites.first());
Map.addLayer(check, {min: 0, max: 1}, 'check');

For the search robots, this is a "temporally grouped reduction."

[GIS] Maximum ndvi for 3 months average in Google Earth Engine

You might need some cloud masking in there, too, but here's the three-monthly part. Also note that the output imagery has three bands: mean, min, and max. You can now use that collection to make a chart, export a table, etc.

var ndvi = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR').map(function(image) {
  return image.select().addBands(image.normalizedDifference(['B5', 'B4']));
});

/* Creates a collection of mosaics with a given temporal interval.
 *
 * collection - the collection from which to make composites.
 * start - the date of the first composite (either a string or an ee.Date)
 * count - the number of composites to make
 * interval - The time between composites, in units of "units".
 * units - The units of step (day, week, month, year; see ee ee.Date.advance)
 */
var temporalCollection = function(collection, start, count, interval, units) {
  // Create a sequence of numbers, one for each time interval.
  var sequence = ee.List.sequence(0, ee.Number(count).subtract(1));

  var originalStartDate = ee.Date(start);

  return ee.ImageCollection(sequence.map(function(i) {
    // Get the start date of the current sequence.
    var startDate = originalStartDate.advance(ee.Number(interval).multiply(i), units);

    // Get the end date of the current sequence.
    var endDate = originalStartDate.advance(
      ee.Number(interval).multiply(ee.Number(i).add(1)), units);

    return collection.filterDate(startDate, endDate)
        .reduce(ee.Reducer.mean().combine({
          reducer2: ee.Reducer.minMax(),
          sharedInputs: true
        }));
  }));
};

var threeMonthlyNDVI = temporalCollection(ndvi, ee.Date('2015-01-01'), 12, 3, 'month');

var check = ee.Image(threeMonthlyNDVI.first());
Map.addLayer(check, {bands: 'nd_mean', min: 0, max: 1}, 'check')

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