Image Collection Statistics in Google Earth Engine?

Question

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']);
});

Answer 1

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!