zonal: Zonal statistics

In raster: Geographic Data Analysis and Modeling

Description

Compute zonal statistics, that is summarized values of a Raster* object for each "zone" defined by a RasterLayer.

If stat is a true function, zonal will fail (gracefully) for very large Raster objects, but it will in most cases work for functions that can be defined as by a character argument ('mean', 'sd', 'min', 'max', or 'sum'). In addition you can use 'count' to count the number of cells in each zone (only useful with na.rm=TRUE, otherwise freq(z) would be more direct.

If a function is used, it should accept a na.rm argument (or at least a ... argument)

Usage

## S4 method for signature 'RasterLayer,RasterLayer'
zonal(x, z, fun='mean', digits=0, na.rm=TRUE, ...) 

## S4 method for signature 'RasterStackBrick,RasterLayer'
zonal(x, z, fun='mean', digits=0, na.rm=TRUE, ...) 

Arguments

x	Raster* object
z	RasterLayer with codes representing zones
fun	function to be applied to summarize the values by zone. Either as character: 'mean', 'sd', 'min', 'max', 'sum'; or, for relatively small Raster* objects, a proper function
digits	integer. Number of digits to maintain in 'zones'. By default averaged to an integer (zero digits)
na.rm	logical. If TRUE, NA values in x are ignored
...	additional arguments. One implemented: progress, as in writeRaster

Value

A matrix with a value for each zone (unique value in zones)

See Also

See cellStats for 'global' statistics (i.e., all of x is considered a single zone), and extract for summarizing values for polygons

Examples

r <- raster(ncols=10, nrows=10)
values(r) <- runif(ncell(r)) * 1:ncell(r)
z <- r
values(z) <- rep(1:5, each=20)
# for large files, use a character value rather than a function
zonal(r, z, 'sum')

# for smaller files you can also provide a function
## Not run: 
zonal(r, z, mean)
zonal(r, z, min)

## End(Not run)

# multiple layers
zonal(stack(r, r*10), z, 'sum')

Example output

Loading required package: sp
     zone      sum
[1,]    1 107.6314
[2,]    2 275.7102
[3,]    3 492.6915
[4,]    4 716.0040
[5,]    5 899.6109
     zone     value
[1,]    1  5.381572
[2,]    2 13.785509
[3,]    3 24.634573
[4,]    4 35.800201
[5,]    5 44.980546
     zone      value
[1,]    1 0.01778881
[2,]    2 1.72120588
[3,]    3 2.22054229
[4,]    4 2.87556308
[5,]    5 1.60465714
     zone  layer.1  layer.2
[1,]    1 107.6314 1076.314
[2,]    2 275.7102 2757.102
[3,]    3 492.6915 4926.915
[4,]    4 716.0040 7160.040
[5,]    5 899.6109 8996.109

raster documentation built on Jan. 5, 2021, 3:01 a.m.