R terra rasterization create vertical white lines - r

Question

I have a raster raster_coarse at 0.25 resolution
class : SpatRaster
dimensions : 26, 31, 1 (nrow, ncol, nlyr)
resolution : 0.25, 0.25 (x, y)
extent : -87.75, -80, 24.5, 31 (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84 (EPSG:4326)
source : memory
name : layer1
min value : 0.08839285
max value : 0.11517857
I have another raster raster_fine at finer resolution
class : SpatRaster
dimensions : 2377, 2758, 1 (nrow, ncol, nlyr)
resolution : 0.002777778, 0.002777778 (x, y)
extent : -87.6361, -79.97499, 24.39723, 31.00001 (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84 (EPSG:4326)
source : memory
name : finer_res
min value : 0
max value : 1
Here's what I am trying to do. I want to populate every pixel in raster_fine with the values from raster_coarse. The way I approached this is:
# check the resolution difference
res_drop <- res(raster_coarse)[1]/res(raster_fine)[1] # 89.99999.
# disaggregate the raster_coarse
raster_coarse <- terra::disagg(raster_coarse, fact = res_drop)
# convert to points
raster_coarse_df <- as.data.frame(raster_coarse, xy = T, na.rm = TRUE)
raster_coarse_pts <- terra::vect(raster_coarse_df, geom = c("x", "y"), crs(raster_fine))
# create a blank canvas
canvas <- terra::rast(xmin = ext(raster_fine)[1],
xmax = ext(raster_fine)[2],
ymin = ext(raster_fine)[3],
ymax = ext(raster_fine)[4],
resolution = res(raster_fine),
crs = crs(raster_fine))
# populate the blank canvas with raster_coarse_pts
raster_coarse <- terra::rasterize(x = raster_coarse_pts , y = canvas, field = "layer1", fun = max, touches = T, background = NA)
raster_coarse
class : SpatRaster
dimensions : 2377, 2758, 1 (nrow, ncol, nlyr)
resolution : 0.002777778, 0.002777778 (x, y)
extent : -87.6361, -79.97499, 24.39723, 31.00001 (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84 (EPSG:4326)
source : memory
name : lyr.1
min value : 0.08839285
max value : 0.11517857
plot(raster_coarse)
I am not able to understand why the vertical white lines are coming from?

Answer 1

Example data
library(terra)
r <- rast(nrow=26, ncol=31, ext=c(-87.75, -80, 24.5, 31))
f <- geodata::gadm("USA", level=1, path=".")
f <- f[f$NAME_1 == "Florida", ]
values(r) <- runif(ncell(r))
r <- mask(r, f)
fine <- rast(nrow=2377, ncol=2758, ext=c(-87.6361, -79.97499, 24.39723, 31.00001))
The solution is to use resample
rr <- resample(r, x, "near")
Instead of the nearest neighbor you could use, for example, bilinear interpolation:
rr <- resample(r, x)
The white lines occur because with your method there will be cells that are not covered. To better understand that you could crop out a small area and plot the points on top of the raster.