Rasters and vectors with terra

Rasters and vectors with terra

This post shows a simple example of how to work with rasters and vectors using the terra package. Terra replaces the older raster package, since terra is usually faster to use.

library(tibble)
library(terra)
library(dplyr)

Then create some objects to work with and plot them.

im1 <- rast(matrix(1:9, nrow = 3),
            crs = "EPSG:4326",
            extent = c(-103,-100,19,22))

pts1 <- vect(data.frame(lon = c(-102.5, -102.5, -100.5, -100.5),
                        lat = c(19.5, 21.5, 21.5, 19.5)),
             geom = c("lon", "lat"), 
             crs = "EPSG:4326")

poly1 <- vect("POLYGON ((-102.5 19.5, -102.5 21.5, -100.5 21.5, -100.5 19.5, -102.5 19.5))",
             crs = "EPSG:4326")

plot(im1)
plot(pts1, add = T)
plot(poly1, add = T)

Data.

Vector operations

Buffer

poly2 <- buffer(poly1, width = 10000, capstyle = "square")
plot(poly2)
plot(poly1, add = T)

Buffer

Intersection

poly3 <- intersect(poly2, poly1)
plot(poly3[[1]])

Intersection

Raster operations

Mask values

im2 <- im1
im2[im2>=5] <- NA
plot(im2)

Masked raster

Operations over all cells

# Stack same image
im3 <- c(im1, im1)

im4 <- app(im3, fun = "sum")
plot(im4)

Sum of both bands

Global operations

global(im1, fun = "mean")
      mean
lyr.1    5

Focal operations

im5 <- focal(im1, w = 3, fun = "max")
plot(im5)

Focal max.

Raster vector operations

Crop

im1_c <- crop(im1, poly1)
plot(im1_c)

Cropped images.

Mask

im2_c <- mask(im1, poly1)
plot(im2_c)

Masked image (seems nothing happened due to overlap between raster and polygon).

Extract values

Manual colors

expts <- extract(im1, pts1)

# Get x and y coordinates and value
geom(pts1) |>
  as_tibble() |>
  select(x, y) |>
  mutate(value = expts|>pull(lyr.1))

# A tibble: 4 × 3
      x     y value
  <dbl> <dbl> <int>
1 -102.  19.5     3
2 -102.  21.5     1
3 -100.  21.5     7
4 -100.  19.5     9