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));
