Adjacent cells — adjacent • raster

Identify cells that are adjacent to a set of cells on a raster.

# S4 method for class 'BasicRaster'
adjacent(x, cells, directions=4, pairs=TRUE, target=NULL, sorted=FALSE, 
         include=FALSE, id=FALSE, ...)

Arguments

x: Raster* object
cells: vector of cell numbers for which adjacent cells should be found. Cell numbers start with 1 in the upper-left corner and increase from left to right and from top to bottom
directions: the number of directions in which cells should be connected: 4 (rook's case), 8 (queen's case), 16 (knight and one-cell queen moves), or 'bishop' to connect cells with one-cell diagonal moves. Or a neighborhood matrix (see Details)
pairs: logical. If TRUE, a matrix of pairs of adjacent cells is returned. If FALSE, a vector of cells adjacent to cells is returned
target: optional vector of target cell numbers that should be considered. All other adjacent cells are ignored
sorted: logical. Should the results be sorted?
include: logical. Should the focal cells be included in the result?
id: logical. Should the id of the cells be included in the result? (numbered from 1 to length(cells)
...: additional arguments. None implemented

Details

A neighborhood matrix identifies the cells around each cell that are considered adjacent. The matrix should have one, and only one, cell with value 0 (the focal cell); at least one cell with value 1 (the adjacent cell(s)); All other cells are not considered adjacent and ignored.

Value

matrix or vector with adjacent cells.

Author

Robert J. Hijmans and Jacob van Etten

Examples

r <- raster(nrows=10, ncols=10)
adjacent(r, cells=c(1, 55), directions=8, pairs=TRUE) 
#>       from to
#>  [1,]   55 44
#>  [2,]    1 10
#>  [3,]   55 54
#>  [4,]    1 20
#>  [5,]   55 64
#>  [6,]   55 46
#>  [7,]    1  2
#>  [8,]   55 56
#>  [9,]    1 12
#> [10,]   55 66
#> [11,]   55 45
#> [12,]    1 11
#> [13,]   55 65

a <- adjacent(r, cell = c(1,55,90), directions=4, sorted=TRUE) 
a
#>       from  to
#>  [1,]    1   2
#>  [2,]    1  10
#>  [3,]    1  11
#>  [4,]   55  45
#>  [5,]   55  54
#>  [6,]   55  56
#>  [7,]   55  65
#>  [8,]   90  80
#>  [9,]   90  81
#> [10,]   90  89
#> [11,]   90 100

r[c(1,55,90)] <- 1
r[a] <- 2
plot(r)


# same result as above
rook <- matrix(c(NA, 1, NA, 
                  1, 0,  1, 
                 NA, 1, NA), ncol=3, byrow=TRUE)

adjacent(r, cells = c(1,55,90), directions=rook, sorted=TRUE) 
#>       from  to
#>  [1,]    1   2
#>  [2,]    1  10
#>  [3,]    1  11
#>  [4,]   55  45
#>  [5,]   55  54
#>  [6,]   55  56
#>  [7,]   55  65
#>  [8,]   90  80
#>  [9,]   90  81
#> [10,]   90  89
#> [11,]   90 100


# Count the number of times that a cell with a certain value
# occurs next to a cell with a certain value
set.seed(0)
r <- raster(ncol=10, nrow=10)
values(r) <- round(runif(ncell(r)) * 5)
a <- adjacent(r, 1:ncell(r), 4, pairs=TRUE)
tb <- table(r[a[,1]], r[a[,2]])
tb
#>    
#>      0  1  2  3  4  5
#>   0  2  4  7  4 10  0
#>   1  4 14 14  7 19  6
#>   2  7 14 30 15 35  8
#>   3  4  7 15 14 13  3
#>   4 10 19 35 13 20  5
#>   5  0  6  8  3  5  0
# make a matrix out of the 'table' object
tb <- unclass(tb)
plot(raster(tb, xmn=-0.5, xmx=5.5, ymn=-0.5, ymx=5.5))