3

How can I extarct mean of each image collection in google earth engine? for example for this code:

var modis = ee.ImageCollection("MODIS/006/MOD13Q1")
.filterBounds(geometry)
.filterDate("2000-01-01","2001-01-01")
.select("NDVI");

print(modis)

var mod13 = modis.map(function(img){
  return img.multiply(0.0001)
  .copyProperties(img,['system:time_start','system:time_end']);
});
Kersten
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AmirHossein Ahrari
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1 Answers1

3

Earth Engine provides very nice functionality to calculate statistics on imagery called reducers. There are plenty of ways to use reducers but as simple examples you can calculate the statistics at the pixel level or provide a geometry to calculate statistics:

var modis = ee.ImageCollection("MODIS/006/MOD13Q1")
.filterDate("2000-01-01","2001-01-01")
.select("NDVI");

print(modis)

var mod13 = modis.map(function(img){
  return img.multiply(0.0001)
  .copyProperties(img,['system:time_start','system:time_end']);
});

// calculate the mean of value for each pixel
var meanMod13 = mod13.reduce(ee.Reducer.mean())
Map.addLayer(meanMod13,{min:0,max:1},'Mean NDVI')

// calculate the mean value for a region
var geom = ee.Geometry.Rectangle([-88,34,-87,35])
Map.addLayer(geom)
// *Note that reduceRegion works only on ee.Image not ee.ImageCollection data types
var zonalStats = meanMod13.reduceRegion({
    geometry: geom,
    reducer: ee.Reducer.mean(),
    scale: 1000,
    bestEffort: true
});
print(zonalStats)

I hope this helps!

Kel Markert
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  • How could I use the mean per region for all images of an ImageCollection? So it automatically does it and gives me the results in a list. With a function? – xdsccc Mar 28 '21 at 09:04
  • Yes, you can create a function that takes an image as input, applies the reduction and outputs the statistics as a feature so the output will be a feature collection. The function will need to be mapped over the image collection and you can get the list of values out of the feature collection. – Kel Markert Mar 29 '21 at 19:41