When your imagery or array data easily fits a couple of times in R’s working memory (RAM), consider yourself lucky. This document was not written for you. If your imagery is too large, or for other reasons you want to work with smaller chunks of data than the files in which they come, read on about your options. First, we will discuss the low-level interface for this, then the higher level, using stars proxy objects that delay all reading.

Preamble: the starsdata package

To run all of the examples in this vignette, you must install a package with datasets that are too large (1 Gb) to be held in the stars package. They are in a drat repo, installation is done by

install.packages("starsdata", repos = "http://pebesma.staff.ifgi.de", type = "source") 

Reading chunks, change resolution, select bands

read_stars has an argument called RasterIO which controls how a GDAL dataset is being read. By default, all pixels and all bands are read in memory. This can consume a lot of time and require a lot of memory. Remember that your file may be compressed, and that pixel values represented in the file by bytes are converted to 8-byte doubles in R.

The reason for using RasterIO for this is that the parameters we use are directly mapped to the GDAL RasterIO function used (after adapting the 1-based offset index in R to 0-based offset in C++).

Reading at a different resolution

Reading datasets at a lower (but also higher!) resolution can be done by setting nBufXSize and nBufYSize

and we see that in addition:

  • the delta (raster cell size) values have increased a factor 5, because nBufXSize and nBufYSize were set to values a factor 5 smaller than nXSize and nYSize
  • the offset coordinates of the grid are still the same
  • the from and to reflect the new area, but relate to the new delta cell size values

We can also read at higher resolution; here we read a 3 x 3 area and blow it up to 100 x 100:

rasterio = list(nXOff = 6, nYOff = 6, nXSize = 3, nYSize = 3,
   nBufXSize = 100, nBufYSize = 100, bands = 1)
x = read_stars(tif, RasterIO = rasterio)
##   x   y 
## 100 100

The reason we “see” only three grid cells is that the default sampling method is “nearest neighbour”. We can modify this by

The following methods are allowed for parameter resample:

resample method used
nearest_neighbour Nearest neighbour (default)
bilinear Bilinear (2x2 kernel)
cubic Cubic Convolution Approximation (4x4 kernel)
cubic_spline Cubic B-Spline Approximation (4x4 kernel)
lanczos Lanczos windowed sinc interpolation (6x6 kernel)
average Average
mode Mode (selects the value which appears most often of all the sampled points)
Gauss Gauss blurring

All these methods are implemented in GDAL; for what these methods exactly do, we refer to the GDAL documentation or source code.

Stars proxy objects

Stars proxy objects take another approach: upon creation they contain no data at all, but only pointers to where the data can be read. Data is only read when it is needed, and only as much as is needed: if we plot a proxy objects, the data are read at the resolution of pixels on the screen, rather than at the native resolution, so that if we have e.g. a 10000 x 10000 Sentinel 2 (level 1C) image, we can open it by

and this happens instantly, because no data is read. When we plot this object,

##    user  system elapsed 
##   1.540   0.604   1.384

This takes only around 1 second, since only those pixels are read that can be seen on the plot. If we read the entire image in memory first, as we would do with

p = read_stars(s2, proxy = FALSE)

then only the reading would take over a minute, and require 5 Gb memory.

Select attributes

We can select attributes as with regular stars objects, by using the first argument to [:

x = c("avhrr-only-v2.19810901.nc",
file_list = system.file(paste0("netcdf/", x), package = "starsdata")
y = read_stars(file_list, quiet = TRUE, proxy = TRUE)
## [1] "sst"  "anom" "err"  "ice"
## stars_proxy object with 1 attribute in files:
## $sst
## [1] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810901.nc\":sst"
## [2] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810902.nc\":sst"
## [3] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810903.nc\":sst"
## [4] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810904.nc\":sst"
## [5] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810905.nc\":sst"
## [6] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810906.nc\":sst"
## [7] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810907.nc\":sst"
## [8] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810908.nc\":sst"
## [9] "NETCDF:\"/usr/local/lib/R/site-library/starsdata/netcdf/avhrr-only-v2.19810909.nc\":sst"
## dimension(s):
##      from   to         offset  delta  refsys point values    
## x       1 1440              0   0.25      NA    NA   NULL [x]
## y       1  720             90  -0.25      NA    NA   NULL [y]
## zlev    1    1          0 [m]     NA      NA    NA   NULL    
## time    1    9 1981-09-01 UTC 1 days POSIXct    NA   NULL

Note that this selection limits the reading from 4 to 1 subdataset from all 9 NetCDF files.

Select an area

Another possibility is to crop, or select a rectangular region based on a spatial object. This can be done by passing a bbox object, or an sf, sfc or stars object from which the bounding box will be taken. An example:

bb = st_bbox(c(xmin = 10.125, ymin = 0.125, xmax = 70.125, ymax = 70.125))
ysub = y[bb]
##      from  to         offset  delta  refsys point values    
## x      41 281              0   0.25      NA    NA   NULL [x]
## y      80 360             90  -0.25      NA    NA   NULL [y]
## zlev    1   1          0 [m]     NA      NA    NA   NULL    
## time    1   9 1981-09-01 UTC 1 days POSIXct    NA   NULL
class(ysub) # still no data here!!
## [1] "stars_proxy" "stars"
plot(ysub, reset = FALSE) # plot reads the data, at resolution that is relevant
plot(st_as_sfc(bb), add = TRUE, lwd = .5, border = 'red')

Lazy evaluation, changing evaluation order

Some other actions can be carried out on stars_proxy objects, but their effect is delayed until the data are actually needed (plot, write_stars). For instance, range selections on dimensions other than shown above first need data, and can only then be carried out. Such functions are added to the object, in an attribute called call_list:

Doing this allows for optimizing the order in which operations are done. As an example, for st_apply, reading can be done sequentially over the dimensions over which the function is applied:

  • If for example a function is applied to each band (such as: compute band quantiles), bands can be read sequentially, and discarded after the quantiles have been computed.
  • If a time series function is applied to pixel time series and the result is plotted on a map, the time series function has only to be evaluated on the pixels actually plotted. This means that in
plot(st_apply(x, c("x", "y"), range))

the order of evaluation needs to be reversed: only plot knows which pixels are going to be shown, so that should be in control of how x is subsampled before st_apply is carried out on this subsample.