writeValues: Write values to a file

Description

Functions for writing blocks (>= 1 row(s)) of values to files. If you want to write all values of a Raster* object at once, you can also use writeRaster which is easier to use but more limited. The functions described here allow writing values to file using chunks of different sizes (e.g. 1 or 10 rows). Function blockSize can be used to suggest chunk size.

Usage

writeStart(x, filename, ...)
writeValues(x, v, ...)
writeStop(x)

Arguments

Raster* object

filename

Output filename

vector (RasterLayer) or matrix (RasterBrick) of values

...

Additional arguments. See under Methods

Value

RasterLayer or RasterBrick object.

Methods

Additional arguments for writeStart rll{ format Character. Output file type. See writeFormats datatype Character. Output data type. See dataType overwrite Logical. If TRUE, "filename" will be overwritten if it exists }

Details

First open a file with writeStart, then write values to it in chunks. When writing is done close the file with writeStop. blockSize can help set the number of rows of each chunk and it returns the row-numbers from where to start reading. When writing a file to disk, the file format is determined by the 'format=' argument if supplied, or else by the file extension (if the extension is known). If other cases the default format is used. The 'factory-fresh' default format is 'raster', but this can be changed using setOptions. Files with a 'native' format must be written in sequence of cell numbers. E.g. when writing by row, you must write all rows, and you must write them in sequence. Start at row=1, and end at row=nrow(raster). In contrast, the values of files written via rgdal can be written in any order (but you must write 1 or more entire rows, you cannot write part of a row). For files written via rgdal, an argument start must be provided to writeValues so that it knows where to write the values. This argument is ignored for the native formats (values are assumed to be in sequence). You cannot overwrite an isolated single row in an existing raster file.

Examples

Run this code

r <- raster(system.file("external/test.grd", package="raster"))
# write to a new binary file in chunks
s <- raster(r)
# 
tr <- blockSize(r)
tr
s <- writeStart(s, filename='test.grd',  overwrite=TRUE)
for (i in 1:tr$n) {
	v <- getValuesBlock(r, row=tr$row[i], nrows=tr$size)
	s <- writeValues(s, v, tr$row[i])
}
s <- writeStop(s)

if(require(rgdal)){
s2 <- writeStart(s, filename='test2.tif', format='GTiff', overwrite=TRUE)
# writing last row first
for (i in tr$n:1) {
	v <- getValuesBlock(r, row=tr$row[i], nrows=tr$size)
	s2 <- writeValues(s2, v, tr$row[i])
}
# row number 5 once more
v <- getValuesBlock(r, row=5, nrows=1)
writeValues(s2, v, 5)
s2 <- writeStop(s2)
}

## write values of a RasterStack to a RasterBrick
s <- stack(system.file("external/rlogo.grd", package="raster"))
# create empty brick
b <- brick(s, values=FALSE)  
b <- writeStart(b, filename="test.grd", format="raster",overwrite=TRUE)
tr <- blockSize(b)
for (i in 1:tr$n) {
	v <- getValuesBlock(s, row=tr$row[i], nrows=tr$size)
	b <- writeValues(b, v, tr$row[i])
}
b <- writeStop(b)
# note that the above is equivalent to 
# b <- writeRaster(s, filename="test.grd", format="raster",overwrite=TRUE)

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