I am using CHIRPS data set (daily precipitation) to derive mean, median, min and max precipitation within a certain time frame. Then, I want to extract the values at specific locations included in a point shapefile and save the results in a table. The script seems to work but the output for the median returns zeros (0) as value. Please see the script below

var lat = 34.35;
var point = ee.Geometry.Point(lat, lng); 
//var aoi = point.buffer(100000); // Create an area (1km buffer around point)
var country = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017')
.filter(ee.Filter.eq('country_co', 'AF'));
var aoi = country;

Map.setCenter(lng, lat, 5); // Center the map on this location, zoom level 10

var start = '2018-02-15'; // initial date of the image collection
var end = '2018-07-15'; //final date of the image collection

var p1 = ee.Geometry.Point([69.78086, 34.65411])
var p2 = ee.Geometry.Point([61.82234, 30.66048])
var table = ee.FeatureCollection(ee.List([ee.Feature(p1),ee.Feature(p2)]))

var dataset = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
    .filterDate(start, end)
var PrMean = dataset.mean().rename('PrMean');

var PrMedian = dataset.median().rename('PrMedian');

var PrMin = dataset.min().rename('PrMin');

var PrMax = dataset.max().rename('PrMax');

var composites = PrMean

var YieldLocations = ee.FeatureCollection(table);

var YPrec = composites.reduceRegions(YieldLocations, ee.Reducer.max(), 1);


1 Answer 1


It looks like your code is working as expected. Try adding the following chart to the bottom of your script:

var chart = ui.Chart.image.seriesByRegion({
    imageCollection: dataset,
    regions: YieldLocations,
    reducer: ee.Reducer.mean()
      interpolateNulls: true,
      lineWidth: 1,
      pointSize: 3,
      title: 'Precip over Time',
      colors: ["ff0000","0000ff"],
      vAxis: {title: 'Precip'},
      hAxis: {title: 'Date', format: 'YYYY-MMM', gridlines: {count: 12}}

You will see that the majority of your precipitation data for both points falls on 0. Because more than half of the values are 0, the median is 0 (for example, in the series 0,0,0,0,0,0,1,3,5,8 the mean is 1.7 but the median is 0).

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