使用R中的ncdf4从URL链接访问netcdf文件
对于ncdf4::nc_open无法从.nc访问某些.nc文件的问题，有什么解决办法吗？我想避免先下载该文件，因为这是一个部署在服务器上的闪亮应用程序，因此我希望避免用户能够将文件下载到服务器。
有些URL可以工作，例如来自THREDDS服务器的这个OPeNDAP URL：
library(ncdf4)
nc <- nc_open("https://dapds00.nci.org.au/thredds/dodsC/uc0/Test_pixel_count.nc")但另一些则没有，例如来自THREDDS服务器的这个NetCDF子集服务URL：
nc <- nc_open("https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1")
# Error in nc_open("https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1") :
# Error in nc_open trying to open file https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1或者这个文件直接从一个网站：
nc <- nc_open("https://www.unidata.ucar.edu/software/netcdf/examples/ECMWF_ERA-40_subset.nc")
# syntax error, unexpected WORD_WORD, expecting SCAN_ATTR or SCAN_DATASET or SCAN_ERROR
# context: <!DOCTYPE^ HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><html><head><title>404 Not Found</title></head><body><h1>Not Found</h1><p>The requested URL was not found on this server.</p></body></html>
# Error in R_nc4_open: NetCDF: file not found
# Error in nc_open("https://www.unidata.ucar.edu/software/netcdf/examples/ECMWF_ERA-40_subset.nc") :
# Error in nc_open trying to open file https://www.unidata.ucar.edu/software/netcdf/examples/ECMWF_ERA-40_subset.nc有什么原因可以解释为什么有些方法起作用，而另一些则不行，还有什么方法可以解决这个问题？
回答 1
Stack Overflow用户
发布于 2021-12-15 17:46:21
我遇到了同样的问题，并联系了ncdf4的创建者。以下是他的答复：
ncdf4 R包是到机器上的基础netcdf库的接口。R包无法避免netcdf库本身的限制或问题。您总是可以看到底层netcdf库是如何通过尝试在URL中使用"ncdump“来处理URL的。例如,
% ncdump "https://dapds00.nci.org.au/thredds/dodsC/uc0/Test_pixel_count.nc"
给予：
netcdf Test_pixel_count {
dimensions:
lat = 283 ;
lon = 451 ;
variables:
byte crs ;
crs:_Unsigned = "false" ;
crs:grid_mapping_name = "latitude_longitude" ;
crs:long_name = "CRS definition" ;
crs:longitude_of_prime_meridian = 0. ;
crs:semi_major_axis = 6378137. ;
crs:inverse_flattening = 298.257222101 ;
crs:spatial_ref = "GEOGCS[\"GEOCENTRIC DATUM of AUSTRALIA\",DATUM[\"GDA94\",SPHEROID[\"GRS80\",6378137,298.257222101]],PRIMEM[\"Greenwich\",0],UNIT[\"degree\",0.0174532925199433],AXIS[\"Longitude\",EAST],AXIS[\"Latitude\",NORTH]]" ;
crs:GeoTransform = "141.4607205456306 0.0075 0 -22.95189554231917 0 -0.0075 " ;例如，netcdf库本身可以读取这个URL/文件。
然而，当我这样做时：
% ncdump "https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1"我知道这个错误：
ncdump "https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1"
ncdump: https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1: https://dapds00.nci.org.au/thredds/ncss/uc0/Test_pixel_count.nc?var=Band1&north=-22.9556&west=142&east=143&south=-25.0706&disableProjSubset=on&horizStride=1:
NetCDF: Malformed or unexpected Constraint因此netcdf库本身无法访问该URL/文件，因此ncdf4无法访问该文件。我无法对ncdf4库做任何事情来弥补这一点。
您可以尝试与netcdf库人员联系，询问这是预期的行为还是bug。
https://stackoverflow.com/questions/68502230
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