Question

我需要用R读取NetCDF文件，并将每个时间步骤导出为平滑的多边形shapefile。我有两个问题：平滑栅格并使用NC文件中的适当投影导出到shapefile。输出是常规网格，不会被投影。

以下是示例代码：

>NCFileName = MyncFile.nc
NCFile = open.ncdf(NCFileName) 
NCFile 
[1] "file CF_OUTPUT.nc has 6 dimensions:"
[1] "time   Size: 61"
[1] "height   Size: 8"
[1] "lat   Size: 185"
[1] "lon   Size: 64"
[1] "Time   Size: 61"
[1] "DateStrLen   Size: 19"
[1] "------------------------"
[1] "file CF_OUTPUT.nc has 20 variables:"
[1] "float temp[lon,lat,height,time]  Longname:Temperature Missval:1e+30"
[1] "float relh[lon,lat,height,time]  Longname:Relative Humidity Missval:1e+30"
[1] "float airm[lon,lat,height,time]  Longname:Air density Missval:1e+30"
[1] "float z[lon,lat,height,time]  Longname:Layer top altitude Missval:1e+30"
[1] "float ZH[lon,lat,height,time]  Longname:Layer top altitude Missval:1e+30"
[1] "float hlay[lon,lat,height,time]  Longname:Layer top altitude Missval:1e+30"
[1] "float PM10ant[lon,lat,height,time]  Longname:PM10ant Concentration Missval:1e+30"
[1] "float PM10bio[lon,lat,height,time]  Longname:PM10bio Concentration Missval:1e+30"
[1] "float PM10[lon,lat,height,time]  Longname:PM10 Concentration Missval:1e+30"
[1] "float PM25ant[lon,lat,height,time]  Longname:PM25ant Concentration Missval:1e+30"
[1] "float PM25bio[lon,lat,height,time]  Longname:PM25bio Concentration Missval:1e+30"
[1] "float PM25[lon,lat,height,time]  Longname:PM25 Concentration Missval:1e+30"
[1] "float C2H4[lon,lat,height,time]  Longname:C2H4 Concentration Missval:1e+30"
[1] "float CO[lon,lat,height,time]  Longname:CO Concentration Missval:1e+30"
[1] "float SO2[lon,lat,height,time]  Longname:SO2 Concentration Missval:1e+30"
[1] "float NO[lon,lat,height,time]  Longname:NO Concentration Missval:1e+30"
[1] "float NO2[lon,lat,height,time]  Longname:NO2 Concentration Missval:1e+30"
[1] "float O3[lon,lat,height,time]  Longname:O3 Concentration Missval:1e+30"
[1] "char Times[DateStrLen,Time]  Longname:Times Missval:NA"
[1] "float HGT[lon,lat,time]  Longname:Topography Missval:1e+30"

nc.a=get.var.ncdf(NCFile , varid = 'NO2', start=c(1,1,1,1), count=c(-1,-1,1,1))
Pol <- rasterToPolygons(raster(nc.a),dissolve = TRUE)
Pol
class       : SpatialPolygonsDataFrame 
features    : 11829 
extent      : 0, 1, 0, 1  (xmin, xmax, ymin, ymax)
coord. ref. : NA 
variables   : 1
names       :             layer 
min values  : 0.219758316874504 
max values  :  0.84041428565979 
writeOGR(Pol, dsn = getwd(), layer = 'testPol', driver = 'ESRI Shapefile', overwrite_layer = TRUE)

然而，我得到的是未投影的多边形格栅。

更新：按照@ kakk11和@RobertH的回答，我能够解决部分问题。我仍然得到一个类似网格的多边形，而不是平滑的。这是我到目前为止所做的：我无法像@RobertH建议的那样直接将变量提取到栅格。所以我使用'get.var.ncdf'然后'raster'：

NCFileName = 'MyncFile.nc'
NCFile = open.ncdf(NCFileName)
nc.a = get.var.ncdf(NCFile, varid = 'NO2', start=c(1,1,1,13), count=c(-1,-1,1,1))
nc.a = raster(nc.a)
# put in correct extent:
lat  = NCFile$dim$lat$vals
lon  = NCFile$dim$lon$vals
ExtentLat = range(lat)
ExtentLon = range(lon)
rm(lat,lon)

nc.a = flip(t(nc.a), direction='y')

# Give it lat/lon coords 
extent(nc.a) = c(ExtentLon,ExtentLat)

然后'cut'命令返回vector，所以我用'ratser：reclassify'：

cuts = c(0,5,15,30,50)
classes <- cbind(cuts[1:length(cuts)-1],cuts[2:length(cuts)],cuts[2:length(cuts)])
nc.class <- reclassify(nc.a, classes)

然后我使用'rasterToPolygons'和'dissolve = TRUE'来创建多边形：

pol <- rasterToPolygons(nc.class, dissolve = TRUE)
# set UTM projection:
WGS84_Projection = "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"
proj4string(pol) <- CRS(WGS84_Projection)
writeOGR(pol, dsn = getwd(), layer = 'file' , driver = 'ESRI Shapefile', overwrite_layer = TRUE)

但是，所有这些都会创建多边形shapefile，其中多边形不平滑，这是主要的挑战。

可以使用一些帮助。

Ilik

Answer 1

首先需要正确创建一个RasterLayer，如下所示：

 r <- raster('MyncFile.nc', var='NO2')
 # or, to get all time steps at once
 # brick('MyncFile.nc', var='NO2')

然后，您可以使用重新分类或剪切来对值进行推广（分类）。例如

 cuts <- seq(0.2, 0.9, 0.1)
 rc <- cut(r, cuts)

制作多边形并保存到shapefile

 pol <- rasterToPolygons(rc, dissolve = TRUE)
 shapefile(pol, 'file.shp')

r读取NetCDF并导出为shapefile

1 个答案: