factors: Categorical rasters

In terra: Spatial Data Analysis

Categorical rasters

Description

A SpatRaster layer can represent a categorical variable (factor). Like factors, SpatRaster categories are stored as integers that have an associated label.

The categories can be inspected with levels and cats. They are represented by a data.frame that must have two or more columns, the first one identifying the (integer) cell values and the other column(s) providing the category labels.

If there are multiple columns with categories, you can set the "active" category to choose the one you want to use.

cats returns the entire data.frame, whereas levels only return two columns: the index and the active category.

To set categories for the first layer of a SpatRaster, you can provide levels<- with a data.frame or a list with a data.frame. To set categories for multiple layers you can provide levels<- with a list with one element (that either has a data.frame or is NULL) for each layer. Use categories to set the categories for a specific layer or specific layers.

droplevels removes categories that are not used (declared but not present as values in the raster) if levels=NULL.

addCats adds additional categories to a layer that already is categorical. It adds new variables, not new levels of an existing categorical variable.

Usage

## S4 method for signature 'SpatRaster'
levels(x)

## S4 replacement method for signature 'SpatRaster'
levels(x)<-value

## S4 method for signature 'SpatRaster'
cats(x, layer)

## S4 method for signature 'SpatRaster'
categories(x, layer=1, value, active=1, ...)

## S4 method for signature 'SpatRaster'
droplevels(x, level=NULL, layer=1)

## S4 method for signature 'SpatRaster'
addCats(x, value, merge=FALSE, layer=1)

Arguments

x	SpatRaster
layer	the layer name or number (positive integer); or 0 for all layers
value	a data.frame (ID, category) that define the categories. Or NULL to remove them
active	positive integer, indicating the column in value to be used as the active category (zero based to skip the first column with the cell values; that is 1 is the second column in value)
level	the categories to remove for the layer specified with layer
merge	logical. If TRUE, the categories are combined with merge using the first column of value as ID. If FALSE the categories are combined with cbind
...	additional arguments (none)

Value

list of data.frames (levels, cats) or logical (is.factor)

See Also

activeCat, catalyze, set.cats, as.factor, is.factor

Examples

set.seed(0)
r <- rast(nrows=10, ncols=10)
values(r) <- sample(3, ncell(r), replace=TRUE)
is.factor(r)

cls <- data.frame(id=1:3, cover=c("forest", "water", "urban"))
levels(r) <- cls
is.factor(r)
r

plot(r, col=c("green", "blue", "light gray"))
text(r, digits=3, cex=.75, halo=TRUE)

# raster starts at 3
x <- r + 2
is.factor(x)

# Multiple categories
d <- data.frame(id=3:5, cover=cls[,2], letters=letters[1:3], value=10:12)
levels(x) <- d
x

# get current index
activeCat(x)
# set index 
activeCat(x) <- 3
activeCat(x)
activeCat(x) <- "letters"
plot(x, col=c("green", "blue", "light gray"))
text(x, digits=3, cex=.75, halo=TRUE)

r <- as.numeric(x)
r

p <- as.polygons(x)
plot(p, "letters", col=c("green", "blue", "light gray"))

terra documentation built on Oct. 13, 2023, 5:08 p.m.