Closed hriihimaki closed 6 years ago
This problem can now be addressed by setting argument small = TRUE
in VeloxRaster_extract
; in this case, small (or oddly shaped) polygons are intersected with the full grid cells, instead of only the grid cell centers.
Applied to your example:
# Reproducible example:
library(velox)
library(raster)
## Make VeloxRaster
mat <- matrix(1:100, 10, 10)
extent <- c(0,1,0,1)
vx <- velox(mat, extent=extent, res=c(0.1,0.1), crs="+proj=longlat +datum=WGS84 +no_defs")
## Make SpatialPolygonsDataFrame
library(sp)
library(rgeos)
set.seed(0)
coords <- cbind(runif(10, extent[1], extent[2]), runif(10, extent[3], extent[4]))
sp <- SpatialPoints(coords)
# Default example
# from https://cran.r-project.org/web/packages/velox/README.html
spol_norm <- gBuffer(sp, width=0.2, byid=TRUE)
spdf_norm <- SpatialPolygonsDataFrame(spol_norm, data.frame(id=1:length(spol_norm)), FALSE)
# Smaller buffer
spol_small<- gBuffer(sp, width=0.05, byid=TRUE)
spdf_small <- SpatialPolygonsDataFrame(spol_small, data.frame(id=1:length(spol_small)), FALSE)
plot(spdf_norm); par(new=F)
plot(spdf_small)
## Extract values and calculate mean, see results
(ex.mat.norm <- vx$extract(spdf_norm, fun=median))
(ex.mat.small <- vx$extract(spdf_small, fun=median, small = TRUE)) # -> No NA values anymore.
I noticed this issue when I was extracting raster data for 6 m radius plots with raster resolution from 1 to 30 meters. At the coarser resolution some of the polygons do not coincide with any pixel centers. Thus, the function (e.g. mean or median) returns NA: