GEE：（二）对影像进行重采样

在使用GEE对影像进行重采样时，主要用到.reproject(crs, crsTransform, scale)函数，下面对该函数的参数以及使用进行介绍。
img.reproject(crs, crsTransform, scale)
this:image (Image):
需要重投影的影像
crs (Projection):
重投影时的坐标系
crsTransform (List, default: null):
The list of CRS transform values. This is a row-major ordering of the 3x2 transform matrix. This option is mutually exclusive with the scale option, and replaces any transform already on the projection.
scale (Float, default: null):
在重投影时可以对分辨率进行设置

这里将Sentinel-2影像重采样到100m作为例子，在GEE中实现代码如下：

//去云函数
function maskS2clouds(image) {
  var qa = image.select('QA60');

  // Bits 10 and 11 are clouds and cirrus, respectively.
  var cloudBitMask = 1 << 10;
  var cirrusBitMask = 1 << 11;

  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0).and(qa.bitwiseAnd(cirrusBitMask).eq(0));

  return image.updateMask(mask).divide(10000);
}

//导入多边形区域
var geometry = 
    ee.Geometry.Polygon(
        [[[17.86466765871711, 51.805755132803185],
          [17.86466765871711, 48.51354071088587],
          [24.85197234621711, 48.51354071088587],
          [24.85197234621711, 51.805755132803185]]], null, false);

//导入哨兵2数据
var image = ee.ImageCollection('COPERNICUS/S2_SR')
                  .filterDate('2017-02-01', '2017-11-30')
                  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE',20))
                  .map(maskS2clouds)
                  .median()
                  .clip(geometry);

//输出未重投影时的坐标系以及分辨率
print('Proj and transform info of image:', image.projection().getInfo())
print('Pixel size in meters:', image.projection().nominalScale().getInfo())

//输出重投影后的坐标系以及分辨率，这里将分辨率设置为100m
var reprojected = image.reproject('EPSG:4326',null,100); 
print('Proj and transform info after reprojection:', reprojected.projection());
print('Pixel size in meters:', reprojected.projection().nominalScale()); 

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结果：

上述方法其实是对原始影像数据进行重投影，在重投影时将分辨率重新设置了，在官网还给了一种重采样的方法，使用的是.resample()函数，具体实现代码如下：

// 加载landsat影像
var landsat = ee.Image('LANDSAT/LC08/C01/T1_TOA/LC08_044034_20160323');

// 设置展示参数
Map.setCenter(-122.37383, 37.6193, 15);
var visParams = {bands: ['B4', 'B3', 'B2'], max: 0.3};

//一般最开始加载影像时默认使用的是最邻近像元插值
Map.addLayer(landsat, visParams, 'original image');

// 对其采用双三次曲线重新采样
var resampled = landsat.resample('bicubic');

// 展示重采样后的数据
Map.addLayer(resampled, visParams, 'resampled');
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如果在实际当中，我们有10m和500m的数据，需要统一尺度，就需要对10m的影像进行下采样，可结合.reduceResolution和.reproject实现：

// Load a MODIS EVI image.
var modis = ee.Image(ee.ImageCollection('MODIS/006/MOD13A1').first())
    .select('EVI');

// Display the EVI image near La Honda, California.
Map.setCenter(-122.3616, 37.5331, 12);
Map.addLayer(modis, {min: 2000, max: 5000}, 'MODIS EVI 500 m');

// Get information about the MODIS projection.
var modisProjection = modis.projection();
print('MODIS projection:', modisProjection);

// Load and display forest cover data at 30 meters resolution.
var forest = ee.Image('UMD/hansen/global_forest_change_2015')
    .select('treecover2000');
Map.addLayer(forest, {max: 80}, 'forest cover 30 m');

// Get the forest cover data at MODIS scale and projection.
var forestMean = forest
    // Force the next reprojection to aggregate instead of resampling.
    .reduceResolution({
      reducer: ee.Reducer.mean(),
      maxPixels: 1024
    })
    // Request the data at the scale and projection of the MODIS image.
    .reproject({
      crs: modisProjection
    });

// Display the aggregated, reprojected forest cover data.
Map.addLayer(forestMean, {max: 80}, 'forest cover at MODIS scale 500 m');
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