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57 lines
2.2 KiB
R
57 lines
2.2 KiB
R
% Generated by roxygen2: do not edit by hand
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% Please edit documentation in R/chunk_apply.R
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\name{chunk_apply}
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\alias{chunk_apply}
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\title{Apply an R function on chunks of a data cube}
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\usage{
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chunk_apply(cube, f)
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}
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\arguments{
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\item{cube}{source data cube}
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\item{f}{R function to apply over all chunks}
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}
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\value{
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a proxy data cube object
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}
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\description{
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Apply an R function on chunks of a data cube
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}
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\details{
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This function internally creates a gdalcubes stream data cube, which streams
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data of a chunk to a new R process. For reading data, the function typically
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calls \code{x <- read_chunk_as_array()} which then results in a 4 dimensional (band, time, y, x) array.
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Similarly \code{write_chunk_from_array(x)} will write a result array as a chunk in the resulting data cube.
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The chunk size of the input cube is important to control how the function will be exposed to the data cube. For example,
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if you want to apply an R function over complete pixel time series, you must define the chunk size argument in \code{\link{raster_cube}}
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to make sure that chunk contain the correct parts of the data.
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}
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\note{
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This function returns a proxy object, i.e., it will not start any computations besides deriving the shape of the result.
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}
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\examples{
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\donttest{
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# create image collection from example Landsat data only
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# if not already done in other examples
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if (!file.exists(file.path(tempdir(), "L8.db"))) {
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L8_files <- list.files(system.file("L8NY18", package = "gdalcubes"),
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".TIF", recursive = TRUE, full.names = TRUE)
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create_image_collection(L8_files, "L8_L1TP", file.path(tempdir(), "L8.db"), quiet = TRUE)
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}
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L8.col = image_collection(file.path(tempdir(), "L8.db"))
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v = cube_view(extent=list(left=388941.2, right=766552.4,
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bottom=4345299, top=4744931, t0="2018-01", t1="2018-12"),
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srs="EPSG:32618", nx = 497, ny=526, dt="P1M")
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L8.cube = raster_cube(L8.col, v)
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L8.cube = select_bands(L8.cube, c("B04", "B05"))
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f <- function() {
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x <- read_chunk_as_array()
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out <- reduce_time(x, function(x) {
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cor(x[1,], x[2,], use="na.or.complete", method = "kendall")
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})
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write_chunk_from_array(out)
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}
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L8.cor = chunk_apply(L8.cube, f)
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}
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}
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