Create square polygons from single centre coordinates and area in R

Question

I am having issues plotting true to geographic extent pixels in R. the files come with a list of daily single coordinates and pixel size (area). There is also a Z element separate from this. The data structure looks this way:

 X <- c(1,3,6,7)
 Y <- c(3,2,7,8)
 Z <- c(38,23,12,12)
 Area <- c(32,23,45,67)

The X and Y are in degrees longitude and latitude while the area is in square kilometres. I create the point features easily using:

library(sp)
A <- cbind(X,Y,Z,Area)
B <- SpatialPoints(A)

I plot these easily using the area values to determine the "cex" for plotting. The Z column is intensity and I use these values to determine the colours . How do I create spatial polygons features using the areas for each point in R? I would be using these points to create gridded rasters.

Josh O'Brien · Accepted Answer · 2015-02-23T02:46:24.510

5

This should do the trick:

library(rgeos)      ## for gBuffer()
library(raster)     ## for bind()

ww <- sqrt(B$Area)/2  ## Widths of buffers needed to produce desired areas    

pp <- list()
for(i in seq_along(B)) {
    pp[i] <- gBuffer(B[i], width=ww[i], quadsegs=1, capStyle="SQUARE")
}
PP <- do.call(bind, pp)

## Check that it worked
plot(PP)
plot(B, add=TRUE)
text(B, labels=1:4, adj=c(-1,0), col="red")

enter image description here

edited Feb 23 '15 at 02:46

answered Feb 23 '15 at 02:17

Josh O'Brien

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Thanks Josh. It worked. Where you have do.call(bind,res), I changed it to (bind,pp) for it to work. – Joke O. Feb 23 '15 at 02:43
@JokeO. - Ah, yes. Thanks for catching that. Cheers. – Josh O'Brien Feb 23 '15 at 02:46
Do you have an idea on how to add the Z values to the polygons before converting to raster, please? – Joke O. Feb 23 '15 at 02:50
@JokeO. -- Not before, but afterwards you can just do `SPP <- SpatialPolygonsDataFrame(PP, data.frame(B))`, after which `data.frame(SPP)` etc. will find the attached data. – Josh O'Brien Feb 23 '15 at 02:55

score 0 · Answer 2 · answered Dec 16 '21 at 01:08

Just to update this a bit, here is an sf solution.

Function to create the square buffers

bSquare <- function(x, a) {
  a <- sqrt(a)/2
  return( sf::st_buffer(x, dist = a, nQuadSegs=1, 
                        endCapStyle = "SQUARE") )
}

Some example data

library(sf)
xy <- st_as_sf(data.frame(x = c(1,3,6,7),
               y = c(3,2,7,8), z = c(38,23,12,12),
               area = c(32,23,45,67)),
               coords = c("x", "y"), 
               agr = "constant")

With variable buffer

sb.var <- bSquare(xy, xy$area)
  plot(st_geometry(sb.var))
    plot(st_geometry(xy), pch=20, add=TRUE)

With fixed buffer

sb <- bSquare(xy, 32)
  plot(st_geometry(sb))
    plot(st_geometry(xy), pch=20, add=TRUE)

Create square polygons from single centre coordinates and area in R

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