Extract a random sample of pixels in each image in a image collection in GEE

Question

I want to extract a random sample of pixel values from each image of an image collection and then create a time series from the output. I have created a function to collect an sample of 1000 point from each image as follows.

function sampleImage(image) {

    var param = {numPoints: 1000, 
                  classBand: 'LST',
                  region:polygonCoordList, 
                  scale:30,
                  tileScale: 4
    };
    var SamPix = image.stratifiedSample(param);
    return SamPix;
}
var training = yearlyCol.map(sampleImage);
 print(training)

When running the code I get this error:

mageCollection (Error) Error in map(ID=0): Image.stratifiedSample: The class band must be integer typed.

From the documentation class band should be a string.

https://developers.google.com/earth-engine/apidocs/ee-image-stratifiedsample

I've also tried to go about it by sample random points (code bellow), but it times out after 5min or so

var randomPoints = ee.FeatureCollection.randomPoints(
    {region: meanTemp.geometry(), points: 1000});
    

function sampleImage(image) {
  
    var param = { collection: randomPoints, 
                  scale:30,
                  tileScale: 4
    };
    var SamPix = image.sampleRegions(param);
    return SamPix;
}
var training = yearlyCol.map(sampleImage);
 print(training);

Do you have any recommendations for fixing this error, or maybe an alternative route to get my desire output?

Link to code

Answer 1

The error mentions that the class band must be integer; thus, you need to convert that band to integers (in your OP it was double typed). You just need to add a pair of lines to your createAnnualImage function, round and cast to integer (int).

var createAnnualImage = function(year) {
  var startDate = ee.Date.fromYMD(year, 1, 1);
  var endDate = startDate.advance(1, 'year');
  // Calculate total precipitation
  var mean = LandsatCol.reduce(ee.Reducer.mean()).rename('LST')
                       // round and transform to integer
                       .round()
                       .int();
  return mean.set({
    'system:time_start': startDate.millis(),
    'system:time_end': endDate.millis(),
    'year': year,
  });
};