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目录
1 完整代码
2 运行结果
利用GEE合成NDVI时，如果研究区较大，一个月的影像覆盖不了整个研究区，就会有缺失的地方，还有就是去云之后，有云量的地区变成空值。
所以今天来用一种插值的方法来填补缺失的影像，以NDVI为例，主要实现原理其实就是用前后两个月的NDVI的均值进行填补。
1 完整代码
var roi = table;
Map.centerObject(roi,7)
var styling = {color:"red",fillColor:"00000000"};
Map.addLayer(roi.style(styling),{},"geometry")
var img_normalize = function(img){ var minMax = img.reduceRegion({ reducer:ee.Reducer.minMax(), geometry: roi, scale: 30, maxPixels: 10e13, tileScale: 16 })
var year = img.get('year')
var normalize = ee.ImageCollection.fromImages( img.bandNames().map(function(name){ name = ee.String(name); var band = img.select(name); return band.unitScale(ee.Number(minMax.get(name.cat('_min'))), ee.Number(minMax.get(name.cat('_max')))); }) ).toBands().rename(img.bandNames()); return normalize;}
function maskL457sr(image) {//l57去云// Bit 0 - Fill// Bit 1 - Dilated Cloud// Bit 2 - Unused// Bit 3 - Cloud// Bit 4 - Cloud Shadowvar qaMask = image.select('QA_PIXEL').bitwiseAnd(parseInt('11111', 2)).eq(0);var saturationMask = image.select('QA_RADSAT').eq(0);// Apply the scaling factors to the appropriate bands.var opticalBands = image.select('SR_B.').multiply(0.0000275).add(-0.2);var thermalBand = image.select('ST_B6').multiply(0.00341802).add(149.0);// Replace the original bands with the scaled ones and apply the masks.return image.addBands(opticalBands, null, true).addBands(thermalBand, null, true).updateMask(qaMask).updateMask(saturationMask);
}
/*function maskL8sr(image) {// Bit 0 - Fill// Bit 1 - Dilated Cloud// Bit 2 - Cirrus// Bit 3 - Cloud// Bit 4 - Cloud Shadowvar qaMask = image.select('QA_PIXEL').bitwiseAnd(parseInt('11111', 2)).eq(0);var saturationMask = image.select('QA_RADSAT').eq(0);// Apply the scaling factors to the appropriate bands.var opticalBands = image.select('SR_B.').multiply(0.0000275).add(-0.2);var thermalBands = image.select('ST_B.*').multiply(0.00341802).add(149.0);// Replace the original bands with the scaled ones and apply the masks.return image.addBands(opticalBands, null, true).addBands(thermalBands, null, true).updateMask(qaMask).updateMask(saturationMask);
}*/
var imageCollection = ee.ImageCollection('LANDSAT/LT05/C02/T1_L2').filterBounds(roi);//1111111
var monthCount = ee.List.sequence(0, 11);// 通过图像收集，生成每月NDVI中值图像
var composites = ee.ImageCollection.fromImages(monthCount.map(function(m) {var startMonth = 1; // 从1月开始var startYear = ee.Number(2000); // 1993-1var month = ee.Date.fromYMD(startYear, startMonth, 1).advance(m,'month').get('month');var year = ee.Date.fromYMD(startYear, startMonth, 1).advance(m,'month').get('year')// 按年筛选，然后按月筛选var filtered = imageCollection.filter(ee.Filter.calendarRange({start: year.subtract(1), // 过去两年的平均数end: year,field: 'year'})).filter(ee.Filter.calendarRange({start: month,field: 'month'}));// mask for clouds and then take the median///var composite = filtered.map(maskL457sr).median().clip(roi);return composite.normalizedDifference(['SR_B4', 'SR_B3']).rename('NDVI').set('month', ee.Date.fromYMD(startYear, startMonth, 1).advance(m,'month')).set('system:time_start', ee.Date.fromYMD(startYear, startMonth, 1).advance(m,'month').millis());
}));
print(composites);
var stackCollection = function(collection) {// 创建一个初始图像.var first = ee.Image(collection.first()).select([]);// Write a function that appends a band to an image.var appendBands = function(image, previous) {return ee.Image(previous).addBands(image);};return ee.Image(collection.iterate(appendBands, first));
};
var compos = stackCollection(composites);
print('插值前', compos);// 用上个月和下个月的平均值替换被遮挡的像素
var replacedVals = composites.map(function(image){var currentDate = ee.Date(image.get('system:time_start'));var meanImage = composites.filterDate(currentDate.advance(-2,'month'), currentDate.advance(2, 'month')).mean();//33333333333333333333333max min median// 替换所有被屏蔽的值return meanImage.where(image, image);
});// 将ImageCollection堆叠成一个多波段的光栅，以便下载
var stackCollection = function(collection) {// 创建一个初始图像.var first = ee.Image(collection.first()).select([]);// Write a function that appends a band to an image.var appendBands = function(image, previous) {return ee.Image(previous).addBands(image);};return ee.Image(collection.iterate(appendBands, first));
};
var stacked = stackCollection(replacedVals);
print('stacked image', stacked);
var Vis = {min: -1,max: 1.0,palette: ['FFFFFF', 'CE7E45', 'DF923D', 'F1B555', 'FCD163', '99B718', '74A901','66A000', '529400', '3E8601', '207401', '056201', '004C00', '023B01','012E01', '011D01', '011301'],};
Map.addLayer(compos.select(6), Vis, '插值前');
// .0-11 分别代表1-12个月
Map.addLayer(stacked.select(6), Vis, 'NDVI');//555555555Export.image.toDrive({image: stacked.select(0),//选择导出影像的波段0-11 分别代表1-12个月description: 'NDVI',//选择导出云盘的文件夹名称crs: "EPSG:4326",//坐标系scale: 30,//空间分辨率region: roi,//研究区maxPixels: 1e13,//最大像元个数folder: 'NDVI'
});
2 运行结果
可以看出，填补的效果还是非常明显的。