Check if a point lies on the boundary of two raster cells

Question

I have 25-km raster grid and a point. If I plot the point on the raster, it falls ~on the borderof two adjacent raster cells. I don't how to generate a sample data for this specific case but here's a visualisation of what I am taking about.

Is there any way I can detect for a group of points how many points are cases like this which could like very close to the border of two adjacent raster cells?

Answer 1

You can do that with the function on_border that I show below. Its arguments are SpatRaster r, coordinates x and y, and a tolerance for comparing real numbers.

on_border <- function(r, x, y, tolerance = sqrt(.Machine$double.eps)) {
    v <- h <- (x >= xmin(r)) & (x <= xmax(r)) & (y >= ymin(r)) & (y <= ymax(r))
    v[v] <- ((x[v] - xmin(r)) %% res(r)[1]) < tolerance
    h[h] <- ((y[h] - ymin(r)) %% res(r)[2]) < tolerance
    h | v
}

Illustration

library(terra)
r <- rast(nrow=5, ncol=5, xmin=0, xmax=5, ymin=0, ymax=5, vals=1:25)
x <- c(1, 1.5, 2, 3.5, 4.5)
y <- c(1, 1.5, 2.5, 5, 5.2)

on_border(r, x, y)
#[1]  TRUE FALSE  TRUE  TRUE FALSE

plot(r); lines(r); points(x,y, xpd=TRUE, pch=20, cex=1.5)

To get the distance to the nearest border

to_border <- function(r, x, y) {
    i <- (x >= xmin(r)) & (x <= xmax(r)) & (y >= ymin(r)) & (y <= ymax(r))
    d <- rep(NA, length(i))
    d[i] <- (x[i] - xmin(r)) %% res(r)[1]
    d[i] <- pmin(d[i], (y[i] - ymin(r)) %% res(r)[2])
    d
}

to_border(r, x, y)
#[1] 0.0 0.5 0.0 0.0  NA

In principle, you could also create lines from the raster, and do a relate query like I show below, but that is really inefficient.

pts <- vect(cbind(x, y), crs=crs(r))
rlns <- aggregate(as.lines(r))
relate(pts, rlns, "intersects")
#      [,1]
#[1,]  TRUE
#[2,] FALSE
#[3,]  TRUE
#[4,]  TRUE
#[5,] FALSE

But if you wanted the distance to the border, you may need to do something like this.

distance(pts, rlns)
#     [,1]
#[1,]  0.0
#[2,]  0.5
#[3,]  0.0
#[4,]  0.0
#[5,]  0.2