Why does raster::extract return several values when using a buffer?

Question

I am trying to extract values from a raster based on points. When I run this code

library(raster)

raster::extract(my_raster, my_points, method='simple')

I get one raster value for each point. However, when I try and do the same with the buffer

raster::extract(my_raster, my_points, method='simple', buffer=250)

I get several values per each point. I read the documentation of the function in detail, but I can not understand why do I get several raster values when I draw a buffer around my points, or how to avoid this.

Thank you.

Answer 1

This is condensed from ?extract

library(raster)
r <- raster(ncol=36, nrow=18, vals=1:(18*36))
xy <- cbind(-50, seq(-80, 80, by=20))

## no function 
extract(r, xy[1:3,], buffer=1000000)
#[[1]]
# [1] 586 587 588 589 590 591 592 593 620 621 622 623 624 625 626 627 628 629 630 631
#[[2]]
#[1] 517 518 552 553 554 555
#[[3]]
#[1] 445 446 481 482

## mean works
extract(r, xy[1:3,], buffer=1000000, fun=mean)
#[1] 611.1 541.5 463.5

## but "mean" fails
extract(r, xy[1:3,], buffer=1000000, fun="mean")
#Error in fun(x) : could not find function "fun"