Add graticules to a SpatRaster in terra

I want to map sample locations on a map of Antarctica.

library(terra)
r<-rast("IBCSO.tif")
#IBCSO.tif was downloaded from https://ibcso.org/current_version/
v<-vect(lonlat, crs="+proj=longlat")
p<-project(v, crs(r))
plot(r)
points(p, col="red", pch=20, cex=1)`

gives me the map.

> rast()
class       : SpatRaster 
dimensions  : 180, 360, 1  (nrow, ncol, nlyr)
resolution  : 1, 1  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84

> crs(r)
[1] "PROJCRS[\"WGS 84 / IBCSO Polar Stereographic\",\n    BASEGEOGCRS[\"WGS 84\",\n        DATUM[\"World Geodetic System 1984\",\n            ELLIPSOID[\"WGS 84\",6378137,298.257223563,\n                LENGTHUNIT[\"metre\",1]]],\n        PRIMEM[\"Greenwich\",0,\n            ANGLEUNIT[\"degree\",0.0174532925199433]],\n        ID[\"EPSG\",4326]],\n    CONVERSION[\"IBCSO Polar Stereographic\",\n        METHOD[\"Polar Stereographic (variant B)\",\n            ID[\"EPSG\",9829]],\n        PARAMETER[\"Latitude of standard parallel\",-65,\n            ANGLEUNIT[\"degree\",0.0174532925199433],\n            ID[\"EPSG\",8832]],\n        PARAMETER[\"Longitude of origin\",0,\n            ANGLEUNIT[\"degree\",0.0174532925199433],\n            ID[\"EPSG\",8833]],\n        PARAMETER[\"False easting\",0,\n            LENGTHUNIT[\"metre\",1],\n            ID[\"EPSG\",8806]],\n        PARAMETER[\"False northing\",0,\n            LENGTHUNIT[\"metre\",1],\n            ID[\"EPSG\",8807]]],\n    CS[Cartesian,2],\n        AXIS[\"easting (X)\",north,\n            MERIDIAN[90,\n                ANGLEUNIT[\"degree\",0.0174532925199433]],\n            ORDER[1],\n            LENGTHUNIT[\"metre\",1]],\n        AXIS[\"northing (Y)\",north,\n            MERIDIAN[0,\n                ANGLEUNIT[\"degree\",0.0174532925199433]],\n            ORDER[2],\n            LENGTHUNIT[\"metre\",1]],\n    USAGE[\n        SCOPE[\"Hydrography and nautical charting.\"],\n        AREA[\"Southern hemisphere - south of 50°S onshore and offshore, including Antarctica.\"],\n        BBOX[-90,-180,-50,180]],\n    ID[\"EPSG\",9354]]"

I would like to add a layer of graticules with steps of 5 degrees latitude and 10 degrees longitude on top of this map. How can I do that?

Solution

Here's my take extending the approach provided by falk-env.

You can use sf::st_graticules() to get more controlled graticules. So with that you can control the steps of the longitude and latitude lines independently:


library(terra)
library(dplyr)

# I downsample here the tif for full reproducibilty and speed increase
# url: https://download.pangaea.de/dataset/937574/files/IBCSO_v2_ice-surface_RGB.tif
# 161.9 Mb
# r <- rast("IBCSO_v2_ice-surface_RGB.tif")
# r2 <- spatSample(r, size = 500000, method ="regular", as.raster = TRUE )
# writeRaster(r2, "IBCSO_downsample.tif")


r <- rast("IBCSO_downsample.tif")


# Substitution for `lonlat`
v <- rnaturalearth::ne_countries(country = "Antarctica", type = "countries") |> 
  vect() |> 
  project("EPSG:9354") 

p <- spatSample(v, 30)

# Create graticule object with sf
grat <- sf::st_graticule(lon=seq(-180,180, 10),
                      lat = seq(-90,0, 5),
                      ndiscr = 5000) %>%
  vect() %>%
  project("EPSG:9354") 


# Base plot

plotRGB(r, axes=TRUE, mar=5)
plot(p, col = "red", add = T)
plot(grat, add=TRUE)

If you want the labels to be in longitude/latitude degrees, you may try with ggplot2 + tidyterra. Basically, overlap the grat object and use the scale_x/y_continuous() trick for labelling only:

# Human readable with ggplot2

lims <- as.vector(ext(r))

library(tidyterra)
library(ggplot2)

ggplot() +
  geom_spatraster_rgb(data=r) + 
  geom_spatvector(data=p, color="red") +
  geom_spatvector(data=grat, color=alpha("grey60", 0.5)) +
  coord_sf(xlim = lims[c("xmin", "xmax")],
           ylim = lims[c("ymin", "ymax")]) +
  scale_x_continuous(breaks = seq(-180, 180, by = 10)) +
  scale_y_continuous(breaks = seq(-90, 90, by = 5))