Search code examples
polygonrastermaskterra

maximal and minimal raster mask using polygons

I have a raster and a polygon, and I want to make two masks:

  1. maximal raster: every cell inside the polygon, including all the cells that the polygon boundary touches
  2. minimal raster: every cell inside the polygon where no cell touches the boundary of the polygon.

Using my reproducible example, I can produce the first using terra::mask, and I can produce the second with the aid of terra::extract(... exact=TRUE).

In real life, my raster has the following dimensions, so I cannot use terra::extract with exact=T, as there is not enough memory (in my computer).

enter class : SpatRaster
dimensions  : 42893, 52031, 1  (nrow, ncol, nlyr)
resolution  : 100, 100  (x, y)
extent      : -2253700, 2949400, -5218400, -929100  (xmin, xmax, ymin, ymax)
coord. ref. : GDA94 / Australian Albers (EPSG:3577) 
source(s)   : memory
name        : update_veg 
min value   :      10005 
max value   :    4190069

here is my code that works, so you can see what I mean:

library(terra)
f <- system.file("ex/lux.shp", package="terra")
v <- vect(f)
v <- v[1:2,]
z <- rast(v, resolution=.05, names="test")
values(z) <- 1:ncell(z)

mask_1 <- mask(z, v, touches=T, inverse=F)
plot(mask_1)
plot(v, add=T)

mask_1 plot

# this extract command will not run on my ginormous raster
k <- extract(z, v, ID=TRUE, touches=T, exact=T, cells=T)
k$mini <- k$test
k$mini[k$fraction < 1] <- NA

mask_2 <- z
values(mask_2) <- NA
values(mask_2)[k$cell] <- k$mini

plot(mask_2)
plot(v, add=T)

mask_2 plot

These plots appear as I would like to see them.

If anyone can help me with a method that might not pound my computer to smithereens, I will be highly appreciative.

Solution

  • You can do

    mask_2 <- rasterize(as.lines(v), mask_1, NA, update=TRUE)
plot(mask_2)
lines(v)