Question

我一直在使用Sam Murphy的代码对Google Earth Engine中的Sentinel-2图像进行大气校正。一切顺利，并且对于单个图像运行非常快。我想做的是将以下代码映射到图像集合上：

output = image.select('QA60')
for band in ['B1','B2','B3','B4','B5','B6','B7','B8','B8A','B9','B10','B11','B12']:
    print(band)
    output = output.addBands(surface_reflectance(band))

我认为我需要一个双重映射函数（以避免在这里使用for循环），但是还没有在GEE中看到任何Python示例。

到目前为止，这是我想出的：

def atcorrector(image):
    qa = image.select('QA60')
    bands = ee.List(['B1','B2','B3','B4','B5','B6','B7','B8','B8A','B9','B10','B11','B12'])
    def mapper(bands):
        return qa.map(addBands(surface_reflectance(bands)))
    return qa

ImageCollection.map(atcorrector)

但是，这并不能返回所有波段的图像，因此我觉得我的嵌套函数无法正常工作。我是python的新手，因此感谢所有帮助！

旁注：我已经尝试过Sam的second repository来对图像集进行大气校正，但是它运行得太慢了，我希望使用提议的map函数作为“服务器端”计算，我有大量要处理的图像。

PS：以下是surface_reflectance函数的代码，摘自Sam Murphy的存储库。它调用了他的定制课程之一，称为“大气”。用于大气校正的模型是Py6S模型。

# Package requirement
from Py6S import * 
import datetime
import math
import os
import sys
sys.path.append(os.path.join(os.path.dirname(os.getcwd()),'bin'))
from atmospheric import Atmospheric #Custom-defined class by Sam
import ee
ee.Initialize()

## The Sentinel-2 image collection
studyarea = ee.Geometry.Rectangle(7.8399,59.9273,8.2299,60.1208)#region of interest
S2 = ee.ImageCollection('COPERNICUS/S2').filterBounds(studyarea)\
       .filterDate('2016-06-01', '2016-06-10').sort('system:time_start')

## define metadata
info = S2one.getInfo()['properties']
scene_date = datetime.datetime.utcfromtimestamp(info['system:time_start']/1000)# i.e. Python uses seconds, EE uses milliseconds
solar_z = info['MEAN_SOLAR_ZENITH_ANGLE']
SRTM = ee.Image('USGS/GMTED2010')  # Make sure that your study area is covered by this elevation dataset
alt = SRTM.reduceRegion(reducer=ee.Reducer.mean(), geometry=studyarea.centroid()).get('be75').getInfo() # insert correct name for elevation variable from dataset
km = alt/1000  # i.e. Py6S uses units of kilometers

date = ee.Date(START_DATE)
# the following three variables are called on from the Atmospheric class Sam defined in his GitHub
h2o = Atmospheric.water(studyarea,date).getInfo() 
o3 = Atmospheric.ozone(studyarea,date).getInfo()
aot = Atmospheric.aerosol(studyarea,date).getInfo()

## Create the 6S Object
s = SixS() # Instantiate
# Atmospheric constituents
s.atmos_profile = AtmosProfile.UserWaterAndOzone(h2o,o3)
s.aero_profile = AeroProfile.Continental
s.aot550 = aot

# Earth-Sun-satellite geometry
s.geometry = Geometry.User()
s.geometry.view_z = 0               # always NADIR (I think..)
s.geometry.solar_z = solar_z        # solar zenith angle
s.geometry.month = scene_date.month # month and day used for Earth-Sun distance
s.geometry.day = scene_date.day     # month and day used for Earth-Sun distance
s.altitudes.set_sensor_satellite_level()
s.altitudes.set_target_custom_altitude(km)

# Extract spectral response function for given band name
def spectralResponseFunction(bandname):
    bandSelect = {
        'B1':PredefinedWavelengths.S2A_MSI_01,
        'B2':PredefinedWavelengths.S2A_MSI_02,
        'B3':PredefinedWavelengths.S2A_MSI_03,
        'B4':PredefinedWavelengths.S2A_MSI_04,
        'B5':PredefinedWavelengths.S2A_MSI_05,
        'B6':PredefinedWavelengths.S2A_MSI_06,
        'B7':PredefinedWavelengths.S2A_MSI_07,
        'B8':PredefinedWavelengths.S2A_MSI_08,
        'B8A':PredefinedWavelengths.S2A_MSI_09,
        'B9':PredefinedWavelengths.S2A_MSI_10,
        'B10':PredefinedWavelengths.S2A_MSI_11,
        'B11':PredefinedWavelengths.S2A_MSI_12,
        'B12':PredefinedWavelengths.S2A_MSI_13,
        }
    return Wavelength(bandSelect[bandname])

# Converts top of atmosphere reflectance to at-sensor radiance
def toa_to_rad(bandname):
    ESUN = info['SOLAR_IRRADIANCE_'+bandname]
    solar_angle_correction = math.cos(math.radians(solar_z))# solar exoatmospheric spectral irradiance
    doy = scene_date.timetuple().tm_yday
    d = 1 - 0.01672 * math.cos(0.9856 * (doy-4))# Earth-Sun distance (from day of year)
    multiplier = ESUN*solar_angle_correction/(math.pi*d**2)# conversion factor
    rad = toa.select(bandname).multiply(multiplier)# at-sensor radiance
    return rad

# Calculate surface reflectance from at-sensor radiance given waveband name
def surface_reflectance(bandname):
    s.wavelength = spectralResponseFunction(bandname)  # run 6S for this waveband
    s.run()
    # extract 6S outputs
    Edir = s.outputs.direct_solar_irradiance             #direct solar irradiance
    Edif = s.outputs.diffuse_solar_irradiance            #diffuse solar irradiance
    Lp   = s.outputs.atmospheric_intrinsic_radiance      #path radiance
    absorb  = s.outputs.trans['global_gas'].upward       #absorption transmissivity
    scatter = s.outputs.trans['total_scattering'].upward #scattering transmissivity
    tau2 = absorb*scatter                                #total transmissivity
    # radiance to surface reflectance
    rad = toa_to_rad(bandname)
    ref = rad.subtract(Lp).multiply(math.pi).divide(tau2*(Edir+Edif))
    return ref

Answer 1

要根据需要应用surface_reflectance函数，只需按如下所示修改代码：

def atcorrector(image):
    qa = image.select('QA60')
    for band in ['B1','B2','B3','B4','B5','B6','B7','B8','B8A','B9','B10','B11','B12']:
        print(band)
        qa = qa.addBands(surface_reflectance(band))
    return qa

ImageCollection.map(atcorrector)

如您所见，此代码仅复制您为单个图像制作的代码。 for循环可以在Python API中正常工作（在JavaScript API中，我不建议您使用此循环）。如果您不想使用for循环，只需稍微更改一下代码即可：

def atcorrector(image):
    qa = image.select('QA60')
    bands = ee.List(['B1','B2','B3','B4','B5','B6','B7','B8','B8A','B9','B10','B11','B12'])

    def mapper(band):
        qa = qa.addBands(surface_reflectance(band))
        return band

    bands.map(mapper)
    return qa

ImageCollection.map(atcorrector)

map的{{1}}函数（或任何具有ee.List函数的ee对象）可以代替map循环。

希望这会有所帮助。

Google Earth Engine Python API：具有波段列表的图像收集上的Map函数

1 个答案: