I am using the terra::focal function to fill missing data in a categorical raster. The problem is that the result of the focal function gives a raster with numerical values. I would really like to preserve the original categories, which are formatted like this "1.1.1" (i.e., numbers between points). Is there a way to do this to keep the categories? I have provided a reproducible example below. For your information, here are the dimensions of my actual raster: 258, 600, 1 (nrow, ncol, nlyr).

set.seed(1)

## Build a raster with categories
r <- terra::rast(matrix(1:25, ncol = 5), crs = "EPSG:3857")
as_tbl <- tibble::as_tibble(r, xy = TRUE)
as_tbl$lyr.1 <- as.factor(paste0(sample(1:10, size = dim(as_tbl)[1], replace = TRUE), ".", sample(1:10, size = dim(as_tbl)[1], replace = TRUE), ".", sample(1:10, size = dim(as_tbl)[1], replace = TRUE)))
newrast <- tidyterra::as_spatraster(as_tbl, crs = "EPSG:3857")
print(newrast)

## Replace cells with NA
newrast[2,2] <- NA
newrast[3,4] <- NA
## plot(newrast)

## Appy the focal function
test <- terra::focal(newrast, w = matrix(1, nrow = 3, ncol = 3), fun = "modal", na.policy = "only", na.rm = TRUE)
print(test)
## plot(test)

I am using the terra::focal function to fill missing data in a categorical raster. The problem is that the result of the focal function gives a raster with numerical values. I would really like to preserve the original categories, which are formatted like this "1.1.1" (i.e., numbers between points). Is there a way to do this to keep the categories? I have provided a reproducible example below. For your information, here are the dimensions of my actual raster: 258, 600, 1 (nrow, ncol, nlyr).

set.seed(1)

## Build a raster with categories
r <- terra::rast(matrix(1:25, ncol = 5), crs = "EPSG:3857")
as_tbl <- tibble::as_tibble(r, xy = TRUE)
as_tbl$lyr.1 <- as.factor(paste0(sample(1:10, size = dim(as_tbl)[1], replace = TRUE), ".", sample(1:10, size = dim(as_tbl)[1], replace = TRUE), ".", sample(1:10, size = dim(as_tbl)[1], replace = TRUE)))
newrast <- tidyterra::as_spatraster(as_tbl, crs = "EPSG:3857")
print(newrast)

## Replace cells with NA
newrast[2,2] <- NA
newrast[3,4] <- NA
## plot(newrast)

## Appy the focal function
test <- terra::focal(newrast, w = matrix(1, nrow = 3, ncol = 3), fun = "modal", na.policy = "only", na.rm = TRUE)
print(test)
## plot(test)

• r
• raster
• terra
• focal

1 Answer 1

With these example data

library(terra)
set.seed(1)
r <- terra::rast(nrow=5, ncol=5, vals=sample(c(LETTERS[1:5], NA), 25, replace=TRUE))

You can do

test <- terra::focal(r, w=3, fun="modal", na.policy="only", na.rm=TRUE)
levels(test) <- levels(r)

test
#class       : SpatRaster 
#dimensions  : 5, 5, 1  (nrow, ncol, nlyr)
#resolution  : 72, 36  (x, y)
#extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (CRS84) (OGC:CRS84) 
#source(s)   : memory
#categories  : letter 
#name        : letter 
#min value   :      A 
#max value   :      E 

In "terra" version 1.8-26 (currently the development version), the categories are now kept automatically if you use focal with "fun=modal", "min", "max", or "first". You can install the development version with

install.packages('terra', repos='https://rspatial.r-universe.dev')