GEE问题：image集合中median和first的区别

问题

我是GEE的新手。我正在试图理解两个图像之间的位移。我正在尝试以下例子： - 加载图像 - 手动替换（将图像移动40米） - 使用位移函数计算图像移动了多少。 - 如果一切顺利，我应该后退40米如果我将代码应用于“原始”图像 image 1 = collection.First（）一切正常如果我将代码应用于“中位数”图像 image 1 = collection.median（）它不是！

函数

median()

Reduces an image collection by calculating the median of all values at each pixel across the stack of all matching bands. Bands are matched by name.

通过计算所有匹配波段堆栈中每个像素处所有值的中位数来聚合图像集合。乐队按名称匹配。

Arguments:

this:collection (ImageCollection):

The image collection to reduce.

Returns: Image

first()

Returns the first entry from a given collection.

Arguments:

this:imagecollection (ImageCollection):

The ImageCollection instance.

Returns: Image

代码

代码语言：javascript复制

// area of interest

var geoJSON = {
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "coordinates": [
          [
            [
              12.411401665803425,
              41.68238436943494
            ],
            [
              12.411401665803425,
              41.64142345909477
            ],
            [
              12.463637299444343,
              41.64142345909477
            ],
            [
              12.463637299444343,
              41.68238436943494
            ],
            [
              12.411401665803425,
              41.68238436943494
            ]
          ]
        ],
        "type": "Polygon"
      }
    }
  ]
}
var coords = geoJSON['features'][0]['geometry']['coordinates']
var aoi = ee.Geometry.Polygon(coords)

// load original image

var start_date = ee.Date('2022-01-01')
var end_date = ee.Date('2024-07-31')
var image1 = ee.ImageCollection('COPERNICUS/S2_HARMONIZED')
var image = image1.filter(ee.Filter.bounds(aoi)).select(['B4', 'B3','B2'])

image1 = image1.filter(ee.Filter.date(start_date,end_date)).median() 
print(image1)

image1 = image.filter(ee.Filter.date(start_date,end_date)).first()
print(image1)
// <== if I use first(), then it works!!?
image1 = image1.rename('R', 'G', 'B').clip(aoi)

// create a displacement image, to shift image of 40 m
// this is probably a stupid way of creating such a displacement...

var x = image1.gt(0)
x = x.addBands({srcImg:x.select('R').multiply(40), overwrite:true})
x = x.addBands({srcImg:x.select('G').multiply(0), overwrite:true})

// create a displaced (shifted) image

var image2 = image1.displace(x)
var center = aoi.centroid()
print(center)

//var s2Vis = {
//  min: 0.0,
//  max: 3000,
//  bands: ['R', 'G', 'B'],
//}
//Map.setCenter(12.44,41.66, 9)
//Map.addLayer(image1,s2Vis);
//Map.addLayer(image2,s2Vis);


// compute displacement

var image1RedBand = image1.select('R')
var image2RedBand = image2.select('R')
var displacement = image2RedBand.displacement({
  referenceImage: image1RedBand,
  maxOffset: 50.0,
  patchWidth: 100.0
});

var offset = displacement.select('dx').hypot(displacement.select('dy'))
var theMax = offset.reduceRegion({reducer:ee.Reducer.max(), geometry:aoi, scale:10, bestEffort:true})
var theMin = offset.reduceRegion({reducer:ee.Reducer.min(), geometry:aoi, scale:10, bestEffort:true})
print('max displacement =',theMax.values())
print('min displacement =',theMin.values())