GEE哨兵二号去云实验

最近突然发现哨兵二号的QA60波段，在2022年7月多按照mask掩膜去云的效果不好，可能是因为云量太高等原因。想到了之前见过的代码，于是按照云量分数和SVM进行了去云。可以在上面继续改进。

var imageCollection = ee.ImageCollection("COPERNICUS/S2")

var geometry = ee.Geometry.Polygon(

[[[116.1703785888879, 40.06062269716839],

[116.1703785888879, 39.7065494906677],

[116.67437639162227, 39.7065494906677],

[116.67437639162227, 40.06062269716839]]], null, false);

// Cloud Detection of Sentinel-2 Image Coupled with SVM and Cloud Scoring Algorithm

// Sample image

var StartDate = '2018-10-10';

var EndDate = '2019-10-10';

var img = ee.ImageCollection("COPERNICUS/S2").filterBounds(geometry)

.filterDate(StartDate,EndDate)

.first();

Map.addLayer(img, {bands:['B4','B3','B2'],min:0,max:3000}, 'Original Image');

Map.centerObject(geometry,11);

// QA60 band cloud detection

function QA60_cloud (img){

var qa60 = img.select('QA60').neq(0);

return img.updateMask(qa60).select('QA60');

}

Map.addLayer(QA60_cloud(img), {palette:['orange']}, 'QA60_band Result');

// Cloud Scoring Algorithm

var _cloudScore = function(img) {

var rescale = function(img, exp, thresholds) {

return img.expression(exp, {img: img})

.subtract(thresholds[0]).divide(thresholds[1] - thresholds[0]);

};

var score = ee.Image.constant(1.0);

score = score.min(rescale(img, 'img.blue', [0.1, 0.3]));

score = score.min(rescale(img, 'img.red + img.green + img.blue', [0.2, 0.8]));

score = score.min(rescale(img, 'img.nir + img.swir1 + img.swir2', [0.3, 0.8]));

var ndsi = img.normalizedDifference(['green', 'swir1']);

return score.min(rescale(ndsi, 'img', [0.8, 0.6]));

};

function rmCloudByScore(image, thread) {

var preBands = ["B2","B3","B4","B8","B11","B12"];

var newBands = ['blue','green','red','nir','swir1','swir2'];

var score = _cloudScore(image.select(preBands, newBands));

score = score.multiply(100).byte().rename('cloud');

return image.addBands(score)

//.updateMask(score.gte(thread));

}

function scaleImage(image) {

var time_start = image.get("system:time_start");

image = image.divide(10000);

image = image.set("system:time_start", time_start);

return image;

}

// SVM_CloudScore cloud detection

function SVM_Class (img) {

var cloud_sample = img.select('cloud').gte(60.0).multiply(1.0).rename('cloud_sample');

img = img.addBands(cloud_sample);

print('Image Information',img)

var training = img.stratifiedSample({

numPoints: 30,

classBand: "cloud_sample",

region:geometry,

scale: 30,

geometries: true

});

Map.addLayer(training,{},'SVM Train Sample');

var SVM_classifier = ee.Classifier.libsvm({

kernelType: 'RBF',

gamma: 0.5,

cost: 10

});

var classifier = SVM_classifier.train(training, "cloud_sample");

var svm_cloud = img.classify(classifier).rename('SVM_Cloud_');

img = img.addBands(svm_cloud);

return img.select('SVM_Cloud_').updateMask(img.select('SVM_Cloud_'));

}

// main function

function main() {

var img_Scale = scaleImage(img);

var img_CloudScore_ = rmCloudByScore(img_Scale, 60);

Map.addLayer(img_CloudScore_.updateMask(img_CloudScore_.select('cloud').gt(50.0)).select('cloud'), {palette:['orange']}, "Cloud Score Result");

Map.addLayer(SVM_Class(img_CloudScore_),{palette:['orange']},'SVMcloud Result');

}

main();

去云效果图