Question

我正在使用Google EarthEngine（Python API，但这并不重要）。我有一个ImageCollection，我需要按区域为集合中的每个图像减少。

有没有办法通过对EarthEngine的单一请求获取.reduceRegion的时间序列。到目前为止，我发现.reduceRegion（＆＃39; mean＆＃39;，feature）仅适用于图像。我需要一个与collection.reduceRegion（＆＃39; mean＆＃39;，feature）相当的 - 它不存在 - 目标是获取每个时间步的值列表。

根本问题是，我在创建时间序列时遇到了EE的请求限制（每秒3次）。此外，对每个值发出请求的速度非常慢。

有没有办法为集合构建合适的reducer。由于集合缩减器需要返回图像（请告诉我，如果这是不正确的），我可以想象，例如在输入集合中为每个图像创建一个带的图像，该图像只有一个具有所需值的像素。

感谢您的帮助

Answer 1

这是一种方法。

在此脚本中，您将获得一个没有空值的字典

# -*- coding: utf-8 -*-
"""
Created on Tue Mar 14 11:21:09 2017

@author: Gennadii
"""
import ee
ee.Initialize()

geometry = ee.Geometry.Polygon([[[-71.54365539550781, -43.07340216393553],
          [-71.5484619140625, -43.11050787253287],
          [-71.488037109375, -43.125043167401266],
          [-71.48460388183594, -43.0754084526532]]])

def calcMean(img):
  # gets the mean NDVI for the area in this img
  mean = img.reduceRegion(ee.Reducer.mean(), geometry, 30).get('NDVI')

  # sets the date and the mean NDVI as a property of the image
  return img.set('date', img.date().format()).set('mean', mean)

# Applies calcMean() in the collection
col = ee.ImageCollection("LANDSAT/LC8_L1T_8DAY_NDVI").filterDate("2014-01-01","2014-03-31").map(calcMean)

# Reduces the images properties to a list of lists
values = col.reduceColumns(ee.Reducer.toList(2), ['date', 'mean']).values().get(0)

# Type casts the result into a List
lista = ee.List(values)

# Converts the list of lists to a Dictionaty
means = ee.Dictionary(lista.flatten())
print "Dictionary of means:", means.getInfo()

和你得到的其他脚本也是空值。它们在此脚本中填充-10，但您可以将其更改为您需要的任何内容。它可以是0或字符串。

# -*- coding: utf-8 -*-
"""
Created on Tue Mar 14 11:17:29 2017

@author: Rodrigo E. Principe
"""
import ee
ee.Initialize()

geometry = ee.Geometry.Polygon([[[-71.54365539550781, -43.07340216393553],
          [-71.5484619140625, -43.11050787253287],
          [-71.488037109375, -43.125043167401266],
          [-71.48460388183594, -43.0754084526532]]])

col = ee.ImageCollection("LANDSAT/LC8_L1T_8DAY_NDVI").filterDate("2014-01-01","2014-03-31")

# Initial empty Dictionary
meansIni = ee.Dictionary()

def calcMean(img, first):

  #gets the year of the image
  year = img.date().format()

  #gets the NDVI
  nd = ee.Image(img).reduceRegion(ee.Reducer.mean(),geometry,30).get("NDVI")

  #Checks for null values and fills them with whatever suits you (-10 is just an option)
  ndvi = ee.Algorithms.If(ee.Algorithms.IsEqual(nd, None), -10, nd)

  #fills the Dictionary
  return ee.Dictionary(first).set(year, ndvi)

# Apply calcMean() to the collection
means = ee.Dictionary(col.iterate(calcMean, meansIni))

print "Dictionary of means:", means.getInfo()

Google EarthEngine：获取reduceRegion的时间序列

1 个答案: