combine: Raster overlay for unique combinations

Description

combine() overlays multiple rasters so that a unique ID is assigned to each unique combination of input values. The input raster layers typically have integer data types (floating point will be coerced to integer by truncation), and must have the same projection, extent and cell size. Pixel counts for each unique combination are obtained, and combination IDs are optionally written to an output raster.

Usage

combine(
  rasterfiles,
  var.names = NULL,
  bands = NULL,
  dstfile = NULL,
  fmt = NULL,
  dtName = "UInt32",
  options = NULL,
  quiet = FALSE
)

Value

A data frame with column cmbid containing the combination IDs, column count containing the pixel counts for each combination, and length(rasterfiles) columns named var.names containing the integer values comprising each unique combination.

Arguments

rasterfiles: Character vector of raster filenames to combine.
var.names: Character vector of length(rasterfiles) containing variable names for each raster layer. Defaults will be assigned if var.names are omitted.
bands: Numeric vector of length(rasterfiles) containing the band number to use for each raster in rasterfiles. Band 1 will be used for each input raster if bands are not specified.
dstfile: Character. Optional output raster filename for writing the per-pixel combination IDs. The output raster will be created (and overwritten if it already exists).
fmt: Character. Output raster format name (e.g., "GTiff" or "HFA").
dtName: Character. Output raster data type name. Combination IDs are sequential integers starting at 1. The data type for the output raster should be large enough to accommodate the potential number of unique combinations of the input values (e.g., "UInt16" or the default "UInt32").
options: Optional list of format-specific creation options in a vector of "NAME=VALUE" pairs (e.g., options = c("COMPRESS=LZW") to set LZW compression during creation of a GTiff file).
quiet: Logical scalar. If TRUE, progress bar and messages will be suppressed. Defaults to FALSE.

Details

To specify input raster layers that are bands of a multi-band raster file, repeat the filename in rasterfiles and provide the corresponding band numbers in bands. For example:


rasterfiles <- c("multi-band.tif", "multi-band.tif", "other.tif")
bands <- c(4, 5, 1)
var.names <- c("multi_b4", "multi_b5", "other")

rasterToVRT() provides options for virtual clipping, resampling and pixel alignment, which may be helpful here if the input rasters are not already aligned on a common extent and cell size.

If an output raster of combination IDs is written, the user should verify that the number of combinations obtained did not exceed the range of the output data type. Combination IDs are sequential integers starting at 1. Typical output data types are the unsigned types: Byte (0 to 255), UInt16 (0 to 65,535) and UInt32 (the default, 0 to 4,294,967,295).

Examples

Run this code

evt_file <- system.file("extdata/storml_evt.tif", package="gdalraster")
evc_file <- system.file("extdata/storml_evc.tif", package="gdalraster")
evh_file <- system.file("extdata/storml_evh.tif", package="gdalraster")
rasterfiles <- c(evt_file, evc_file, evh_file)
var.names <- c("veg_type", "veg_cov", "veg_ht")
tbl <- combine(rasterfiles, var.names)
nrow(tbl)
tbl <- tbl[order(-tbl$count),]
head(tbl, n = 20)

# combine two bands from a multi-band file and write the combination IDs
# to an output raster
lcp_file <- system.file("extdata/storm_lake.lcp", package="gdalraster")
rasterfiles <- c(lcp_file, lcp_file)
bands <- c(4, 5)
var.names <- c("fbfm", "tree_cov")
cmb_file <- file.path(tempdir(), "fbfm_cov_cmbid.tif")
opt <- c("COMPRESS=LZW")
tbl <- combine(rasterfiles, var.names, bands, cmb_file, options = opt)
head(tbl)
ds <- new(GDALRaster, cmb_file)
ds$info()
ds$close()
deleteDataset(cmb_file)