raster (version 2.5-8)

rasterize: Rasterize points, lines, or polygons

Description

Transfer values associated with 'object' type spatial data (points, lines, polygons) to raster cells.

For polygons, values are transferred if the polygon covers the center of a raster cell. For lines, values are transferred to all cells that are touched by a line. You can combine this behaviour by rasterizing polygons as lines first and then as polygons.

If x represents points, each point is assigned to a grid cell. Points that fall on a border between cells are placed in the cell to the right and/or in the cell below. The value of a grid cell is determined by the values associated with the points and function fun.

Usage

# S4 method for matrix,Raster
rasterize(x, y, field, fun='last', background=NA,
     mask=FALSE, update=FALSE, updateValue='all', filename="", na.rm=TRUE, ...)
	
# S4 method for SpatialPoints,Raster
rasterize(x, y, field, fun='last', background=NA, 
    mask=FALSE, update=FALSE, updateValue='all', filename="", na.rm=TRUE, ...)
	
# S4 method for SpatialLines,Raster
rasterize(x, y, field, fun='last', background=NA, 
    mask=FALSE, update=FALSE, updateValue='all', filename="", ...)
	
# S4 method for SpatialPolygons,Raster
rasterize(x, y, field, fun='last', background=NA, 
    mask=FALSE, update=FALSE, updateValue='all', filename="", 
    getCover=FALSE, silent=TRUE, ...)

Arguments

x

points (a SpatialPoints* object, or a two-column matrix (or data.frame)), SpatialLines*, SpatialPolygons*, or an Extent object

y

Raster* object

field

numeric or character. The value(s) to be transferred. This can be a single number, or a vector of numbers that has the same length as the number of spatial features (points, lines, polygons). If x is a Spatial*DataFrame, this can be the column name of the variable to be transferred. If missing, the attribute index is used (i.e. numbers from 1 to the number of features). You can also provide a vector with the same length as the number of spatial features, or a matrix where the number of rows matches the number of spatial features

fun

function or character. To determine what values to assign to cells that are covered by multiple spatial features. You can use functions such as min, max, or mean, or one of the following character values: 'first', 'last', 'count'. The default value is 'last'. In the case of SpatialLines*, 'length' is also allowed (currently for planar coordinate systems only).

If x represents points, fun must accept a na.rm argument, either explicitly or through 'dots'. This means that fun=length fails, but fun=function(x,...)length(x) works, although it ignores the na.rm argument. To use the na.rm argument you can use a function like this: fun=function(x, na.rm)if (na.rm) length(na.omit(x)) else (length(x), or use a function that removes NA values in all cases, like this function to compute the number of unique values "richness": fun=function(x, ...) {length(unique(na.omit(x)))} .

If you want to know the number of points in each grid cell, you can use fun='count' or fun=function(x,...){length(x)}. For the number of unique values per grid cell you can use: fun=function(x, ...){ length(unique(na.rm(x)))}. You can also pass multiple functions using a statement like fun=function(x, ...) c(length(x),mean(x)), in which case the returned object is a RasterBrick (multiple layers).

background

numeric. Value to put in the cells that are not covered by any of the features of x. Default is NA

mask

logical. If TRUE the values of the input Raster object are 'masked' by the spatial features of x. That is, cells that spatially overlap with the spatial features retain their values, the other cells become NA. Default is FALSE. This option cannot be used when update=TRUE

update

logical. If TRUE, the values of the Raster* object are updated for the cells that overlap the spatial features of x. Default is FALSE. Cannot be used when mask=TRUE

updateValue

numeric (normally an integer), or character. Only relevant when update=TRUE. Select, by their values, the cells to be updated with the values of the spatial features. Valid character values are 'all', 'NA', and '!NA'. Default is 'all'

filename

character. Output filename (optional)

na.rm

If TRUE, NA values are removed if fun honors the na.rm argument

getCover

logical. If TRUE, the fraction of each grid cell that is covered by the polygons is returned (and the values of field, fun, mask, and update are ignored. The fraction covered is estimated by dividing each cell into 100 subcells and determining presence/absence of the polygon in the center of each subcell

silent

Logical. If TRUE, feedback on the polygon count is suppressed. Default is FALSE

...

Additional arguments for file writing as for writeRaster

Value

RasterLayer or RasterBrick

See Also

extract

Examples

# NOT RUN {
###############################
# rasterize points
###############################
r <- raster(ncols=36, nrows=18)
n <- 1000
x <- runif(n) * 360 - 180
y <- runif(n) * 180 - 90
xy <- cbind(x, y)
# get the (last) indices
r0 <- rasterize(xy, r)
# prensence/absensce (NA) (is there a point or not?)
r1 <- rasterize(xy, r, field=1)
# how many points?
r2 <- rasterize(xy, r, fun=function(x,...)length(x))
vals <- runif(n)
# sum of the values associated with the points
r3 <- rasterize(xy, r, vals, fun=sum)

# with a SpatialPointsDataFrame
vals <- 1:n
p <- data.frame(xy, name=vals)
coordinates(p) <- ~x+y
r <- rasterize(p, r, 'name', fun=min)
#r2 <- rasterize(p, r, 'name', fun=max)
#plot(r, r2, cex=0.5)

###############################
# rasterize lines
###############################
cds1 <- rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60))
cds2 <- rbind(c(-10,0), c(140,60), c(160,0), c(140,-55))
cds3 <- rbind(c(-125,0), c(0,60), c(40,5), c(15,-45))

lines <- spLines(cds1, cds2, cds3)

r <- raster(ncols=90, nrows=45)
r <- rasterize(lines, r)

# }
# NOT RUN {
plot(r)
plot(lines, add=TRUE)

r <- rasterize(lines, r, fun='count')
plot(r)

r[] <- 1:ncell(r)
r <- rasterize(lines, r, mask=TRUE)
plot(r)

r[] <- 1
r[lines] <- 10
plot(r)
# }
# NOT RUN {
###############################
# rasterize polygons
###############################

p1 <- rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60), c(-180,-20))
hole <- rbind(c(-150,-20), c(-100,-10), c(-110,20), c(-150,-20))
p1 <- list(p1, hole)
p2 <- rbind(c(-10,0), c(140,60), c(160,0), c(140,-55), c(-10,0))
p3 <- rbind(c(-125,0), c(0,60), c(40,5), c(15,-45), c(-125,0))

pols <- spPolygons(p1, p2, p3)

r <- raster(ncol=90, nrow=45)
r <- rasterize(pols, r, fun=sum)


# }
# NOT RUN {
plot(r)
plot(pols, add=T)

# add a polygon
p5 <- rbind(c(-180,10), c(0,90), c(40,90), c(145,-10),  
            c(-25, -15), c(-180,0), c(-180,10))
addpoly <- SpatialPolygons(list(Polygons(list(Polygon(p5)), 1)))
addpoly <- as(addpoly, "SpatialPolygonsDataFrame")
addpoly@data[1,1] <- 10
r2 <- rasterize(addpoly, r, field=1, update=TRUE, updateValue="NA")
plot(r2)
plot(pols, border="blue", lwd=2, add=TRUE)
plot(addpoly, add=TRUE, border="red", lwd=2)

# get the percentage cover of polygons in a cell
r3 <- raster(ncol=36, nrow=18)
r3 <- rasterize(pols, r3, getCover=TRUE)
# }