ncdfCF
The ncdfCF package provides an easy to use interface to NetCDF
resources in R, either in local files or remotely on a THREDDS server.
It is built on the RNetCDF package which, like package ncdf4,
provides a basic interface to the netcdf library, but which lacks an
intuitive user interface. Package ncdfCF provides a high-level
interface using functions and methods that are familiar to the R user.
It reads the structural metadata and also the attributes upon opening
the resource. In the process, the ncdfCF package also applies CF
Metadata Conventions to interpret the data. This currently applies to:
- The axis designation. The three mechanisms to identify the axis each dimension represents are applied until an axis is determined.
- The time dimension. Time is usually encoded as an offset from a
datum. Using the
CFtimepackage these offsets can be turned into intelligible dates and times, for all 9 defined calendars. - Bounds information. When present, bounds are read and used in analyses.
- Discrete dimensions with character labels.
Basic usage
Opening and inspecting the contents of a NetCDF resource is very straightforward:
library(ncdfCF)
#>
#> Attaching package: 'ncdfCF'
#> The following object is masked from 'package:graphics':
#>
#> axis
# Get any NetCDF file
fn <- system.file("extdata", "ERA5land_Rwanda_20160101.nc", package = "ncdfCF")
# Open the file, all metadata is read
ds <- open_ncdf(fn)
# Easy access in understandable format to all the details
ds
#> Dataset : /private/var/folders/gs/s0mmlczn4l7bjbmwfrrhjlt80000gn/T/Rtmp4NMDYW/temp_libpath133b9482f742d/ncdfCF/extdata/ERA5land_Rwanda_20160101.nc
#>
#> Variables :
#> id name long_name units dimensions
#> 3 t2m 2 metre temperature K longitude, latitude, time
#> 4 pev Potential evaporation m longitude, latitude, time
#> 5 tp Total precipitation m longitude, latitude, time
#>
#> Dimensions:
#> id axis name dims unlim
#> 1 X longitude [31: 28 ... 31]
#> 2 Y latitude [21: -1 ... -3]
#> 0 T time [24: 2016-01-01 00:00:00 ... 2016-01-01 23:00:00] U
#>
#> Attributes:
#> id name type length value
#> 0 CDI NC_CHAR 64 Climate Data Interface version 2.4.1 ...
#> 1 Conventions NC_CHAR 6 CF-1.6
#> 2 history NC_CHAR 482 Tue May 28 18:39:12 2024: cdo seldate...
#> 3 CDO NC_CHAR 64 Climate Data Operators version 2.4.1 ...
# Variables can be accessed through standard list-type extraction syntax
t2m <- ds[["t2m"]]
t2m
#> Variable: [3] t2m | 2 metre temperature
#>
#> Dimensions:
#> id axis name dims unlim
#> 1 X longitude [31: 28 ... 31]
#> 2 Y latitude [21: -1 ... -3]
#> 0 T time [24: 2016-01-01 00:00:00 ... 2016-01-01 23:00:00] U
#>
#> Attributes:
#> id name type length value
#> 0 long_name NC_CHAR 19 2 metre temperature
#> 1 units NC_CHAR 1 K
#> 2 add_offset NC_DOUBLE 1 292.664569285614
#> 3 scale_factor NC_DOUBLE 1 0.00045127252204996
#> 4 _FillValue NC_SHORT 1 -32767
#> 5 missing_value NC_SHORT 1 -32767
# Same with dimensions, but now without first putting the object in a variable
ds[["longitude"]]
#> Dimension: [1] longitude
#> Axis : X
#> Length : 31
#> Range : 28 ... 31 degrees_east
#> Bounds : (not set)
#>
#> Attributes:
#> id name type length value
#> 0 standard_name NC_CHAR 9 longitude
#> 1 long_name NC_CHAR 9 longitude
#> 2 units NC_CHAR 12 degrees_east
#> 3 axis NC_CHAR 1 X
# Regular base R operations simplify life further
dimnames(ds[["pev"]]) # A variable: list of dimension names
#> longitude latitude time
#> "longitude" "latitude" "time"dimnames(ds[["longitude"]]) # A dimension: vector of dimension element values
#> [1] 28.0 28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 29.0 29.1 29.2 29.3 29.4
#> [16] 29.5 29.6 29.7 29.8 29.9 30.0 30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 30.9
#> [31] 31.0
# Access attributes
attribute(ds[["pev"]], "long_name")
#> [1] "Potential evaporation"Extracting data
There are two ways to read data for a variable from the resource:
[]: The usual R array operator. This uses index values into the dimensions and requires you to know the order in which the dimensions are specified for the variable. With a bit of tinkering and some helper functions inncdfCFthis is still very easy to do.subset(): Thesubset()method lets you specify what you want to extract from each dimension in real-world coordinates and timestamps, in whichever order.
# Extract a timeseries for a specific location
ts <- t2m[5, 4, ]
str(ts)
#> num [1, 1, 1:24] 293 292 292 291 291 ...
#> - attr(*, "dimnames")=List of 3
#> ..$ : chr "28.4"
#> ..$ : chr "-1.3"
#> ..$ : chr [1:24] "2016-01-01 00:00:00" "2016-01-01 01:00:00" "2016-01-01 02:00:00" "2016-01-01 03:00:00" ...
#> - attr(*, "axis")= Named chr [1:3] "X" "Y" "T"
#> ..- attr(*, "names")= chr [1:3] "longitude" "latitude" "time"
#> - attr(*, "time")=List of 1
#> ..$ time:Formal class 'CFtime' [package "CFtime"] with 4 slots
#> .. .. ..@ datum :Formal class 'CFdatum' [package "CFtime"] with 5 slots
#> .. .. .. .. ..@ definition: chr "hours since 1900-01-01 00:00:00.0"
#> .. .. .. .. ..@ unit : int 3
#> .. .. .. .. ..@ origin :'data.frame': 1 obs. of 8 variables:
#> .. .. .. .. .. ..$ year : int 1900
#> .. .. .. .. .. ..$ month : num 1
#> .. .. .. .. .. ..$ day : num 1
#> .. .. .. .. .. ..$ hour : num 0
#> .. .. .. .. .. ..$ minute: num 0
#> .. .. .. .. .. ..$ second: num 0
#> .. .. .. .. .. ..$ tz : chr "+0000"
#> .. .. .. .. .. ..$ offset: num 0
#> .. .. .. .. ..@ calendar : chr "gregorian"
#> .. .. .. .. ..@ cal_id : int 1
#> .. .. ..@ resolution: num 1
#> .. .. ..@ offsets : num [1:24] 1016832 1016833 1016834 1016835 1016836 ...
#> .. .. ..@ bounds : logi FALSE
# Extract the full spatial extent for one time step
ts <- t2m[, , 12]
str(ts)
#> num [1:31, 1:21, 1] 300 300 300 300 300 ...
#> - attr(*, "dimnames")=List of 3
#> ..$ : chr [1:31] "28" "28.1" "28.2" "28.3" ...
#> ..$ : chr [1:21] "-1" "-1.1" "-1.2" "-1.3" ...
#> ..$ : chr "2016-01-01 11:00:00"
#> - attr(*, "axis")= Named chr [1:3] "X" "Y" "T"
#> ..- attr(*, "names")= chr [1:3] "longitude" "latitude" "time"
#> - attr(*, "time")=List of 1
#> ..$ time:Formal class 'CFtime' [package "CFtime"] with 4 slots
#> .. .. ..@ datum :Formal class 'CFdatum' [package "CFtime"] with 5 slots
#> .. .. .. .. ..@ definition: chr "hours since 1900-01-01 00:00:00.0"
#> .. .. .. .. ..@ unit : int 3
#> .. .. .. .. ..@ origin :'data.frame': 1 obs. of 8 variables:
#> .. .. .. .. .. ..$ year : int 1900
#> .. .. .. .. .. ..$ month : num 1
#> .. .. .. .. .. ..$ day : num 1
#> .. .. .. .. .. ..$ hour : num 0
#> .. .. .. .. .. ..$ minute: num 0
#> .. .. .. .. .. ..$ second: num 0
#> .. .. .. .. .. ..$ tz : chr "+0000"
#> .. .. .. .. .. ..$ offset: num 0
#> .. .. .. .. ..@ calendar : chr "gregorian"
#> .. .. .. .. ..@ cal_id : int 1
#> .. .. ..@ resolution: num NA
#> .. .. ..@ offsets : num 1016843
#> .. .. ..@ bounds : logi FALSENote that the results contain degenerate dimensions (of length 1). This by design because it allows attributes to be attached in a consistent manner.
# Extract a specific region, full time dimension
ts <- subset(t2m, list(X = 29:30, Y = -1:-2))
str(ts)
#> num [1:10, 1:10, 1:24] 290 291 291 292 293 ...
#> - attr(*, "dimnames")=List of 3
#> ..$ : chr [1:10] "29" "29.1" "29.2" "29.3" ...
#> ..$ : chr [1:10] "-1" "-1.1" "-1.2" "-1.3" ...
#> ..$ : chr [1:24] "2016-01-01 00:00:00" "2016-01-01 01:00:00" "2016-01-01 02:00:00" "2016-01-01 03:00:00" ...
#> - attr(*, "axis")= Named chr [1:3] "X" "Y" "T"
#> ..- attr(*, "names")= chr [1:3] "longitude" "latitude" "time"
#> - attr(*, "time")=List of 1
#> ..$ time:Formal class 'CFtime' [package "CFtime"] with 4 slots
#> .. .. ..@ datum :Formal class 'CFdatum' [package "CFtime"] with 5 slots
#> .. .. .. .. ..@ definition: chr "hours since 1900-01-01 00:00:00.0"
#> .. .. .. .. ..@ unit : int 3
#> .. .. .. .. ..@ origin :'data.frame': 1 obs. of 8 variables:
#> .. .. .. .. .. ..$ year : int 1900
#> .. .. .. .. .. ..$ month : num 1
#> .. .. .. .. .. ..$ day : num 1
#> .. .. .. .. .. ..$ hour : num 0
#> .. .. .. .. .. ..$ minute: num 0
#> .. .. .. .. .. ..$ second: num 0
#> .. .. .. .. .. ..$ tz : chr "+0000"
#> .. .. .. .. .. ..$ offset: num 0
#> .. .. .. .. ..@ calendar : chr "gregorian"
#> .. .. .. .. ..@ cal_id : int 1
#> .. .. ..@ resolution: num 1
#> .. .. ..@ offsets : num [1:24] 1016832 1016833 1016834 1016835 1016836 ...
#> .. .. ..@ bounds : logi FALSE
# Extract specific time slices for a specific region
# Note that the dimensions are specified out of order and using alternative
# specifications: only the extreme values are used.
ts <- subset(t2m, list(T = c("2016-01-01 09:00", "2016-01-01 15:00"),
X = c(29.6, 28.8),
Y = seq(-2, -1, by = 0.05)))
str(ts)
#> num [1:8, 1:10, 1:6] 297 296 296 298 299 ...
#> - attr(*, "dimnames")=List of 3
#> ..$ : chr [1:8] "28.8" "28.9" "29" "29.1" ...
#> ..$ : chr [1:10] "-1" "-1.1" "-1.2" "-1.3" ...
#> ..$ : chr [1:6] "2016-01-01 09:00:00" "2016-01-01 10:00:00" "2016-01-01 11:00:00" "2016-01-01 12:00:00" ...
#> - attr(*, "axis")= Named chr [1:3] "X" "Y" "T"
#> ..- attr(*, "names")= chr [1:3] "longitude" "latitude" "time"
#> - attr(*, "time")=List of 1
#> ..$ time:Formal class 'CFtime' [package "CFtime"] with 4 slots
#> .. .. ..@ datum :Formal class 'CFdatum' [package "CFtime"] with 5 slots
#> .. .. .. .. ..@ definition: chr "hours since 1900-01-01 00:00:00.0"
#> .. .. .. .. ..@ unit : int 3
#> .. .. .. .. ..@ origin :'data.frame': 1 obs. of 8 variables:
#> .. .. .. .. .. ..$ year : int 1900
#> .. .. .. .. .. ..$ month : num 1
#> .. .. .. .. .. ..$ day : num 1
#> .. .. .. .. .. ..$ hour : num 0
#> .. .. .. .. .. ..$ minute: num 0
#> .. .. .. .. .. ..$ second: num 0
#> .. .. .. .. .. ..$ tz : chr "+0000"
#> .. .. .. .. .. ..$ offset: num 0
#> .. .. .. .. ..@ calendar : chr "gregorian"
#> .. .. .. .. ..@ cal_id : int 1
#> .. .. ..@ resolution: num 6
#> .. .. ..@ offsets : num [1:2] 1016841 1016847
#> .. .. ..@ bounds : logi FALSEBoth of these methods will read data from the NetCDF resource, but only as much as is requested.
Development plan
Package ncdfCF is in the early phases of development. It supports
reading of dimensions, variables, attributes and data from NetCDF
resources in “classic” and “NetCDF4” formats. From the CF Metadata
Conventions it supports identification of dimension axes, interpretation
of the “time” dimension, and reading of “bounds” information.
Development plans for the near future focus on supporting the below features:
NetCDF
- Support for writing.
- Support for “group” information in “NetCDF4” formatted resources.
CF Metadata Conventions
- Full support for discrete or categorical dimensions.
- Interface to “standard_name” libraries and other “defined vocabularies”.
- Compliance with CMIP5 / CMIP6 requirements.
Installation
CAUTION: Package ncdfCF is still
in the early phases of development. While extensively tested on multiple
well-structured datasets, errors may still occur, particularly in
datasets that do not adhere to the CF Metadata Conventions.
Package ncdfCF has not yet been submitted to CRAN.
You can install the development version of ncdfCF from
GitHub with:
# install.packages("devtools")
devtools::install_github("pvanlaake/ncdfCF")