[GIS] Sampling Image collection google earth engine

google-earth-engine

Question: How can I get a random sample of images from an image collection?

Background:
I am working with the GRIDMET: University of Idaho Gridded Surface Meteorological Dataset which provides daily weather for the continental US from 1979-2015 as an image collection. I am interested in calculating the 90th percentile value for one of the weather variables (specifically ERC) at a large number (30,000) points in Washington and Oregon. The GRIDMET image Collection is daily from 1979-present about 13,000 images, obviously calculating the 90th percentile from all the images is taking a long time. I have already clipped the image collection to only the points of interest. I am looking to randomly sample the image collection to speed up my code.

Current Code:

// import Fire Features and GRIDMET image collection
var fires = ee.FeatureCollection("users/brycekellogg/Fires_test");
var Idaho = ee.ImageCollection("IDAHO_EPSCOR/GRIDMET");

Map.setCenter(-119.905, 45.836, 6); // Center on Oregon.
Map.addLayer(fires);// add the 5 test fire points to the map.

var firefeature = ee.FeatureCollection(fires); // convert test fires into a 
feature collection 

var fullERC = Idaho.select('erc')// select only the ERC values

// Create a funtion to clip each image to the feature collection
var clipFeature = function(image) {
  return image.clipToCollection(firefeature);
};

// Map the clip funtion over the Image Collection 
var fullERCclip= fullERC.map(clipFeature);

//Find the 90th percentile of the image collection
var ercPrecentile90 = fullERCclip.reduce(ee.Reducer.percentile({
  percentiles:[90]
}));

// define function to extract 90th percentile image at points
var find90piErc = function(feature) {
 var ercpi90 = ercPrecentile90.reduceRegion({
    reducer:ee.Reducer.first(),
    geometry: feature.geometry(),
    scale:400
  });
  return feature.set({erc90pi:ercpi90.get('erc_p90')
  });
};

// map function on to the fire features
var firefeature_pi_added = firefeature.map(find90piErc);

// print first feature
print('Fire Features with 90th Percentile', firefeature_pi_added);

Here is the solution that I came up with to select 1000 random images:

var fullERCselect = ee.ImageCollection(fullERC.randomColumn('random',13).sort('random').limit(1000));

Best Answer

Here is the solution that I came up with to select 1000 random images:

var fullERCselect = ee.ImageCollection(fullERC.randomColumn('random',13).sort('random').limit(1000));
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