Creating R loop to read in shapefiles from a directory and perform zonal statistics on each

问题

I have 120 county shapefiles in a directory "Counties". I want to use R to read in each shapefile and, for each shapefile, perform zonal statistics (mean) using a single raster layer "NOAA_normal_crop."

I was able to create a script that reads in all of the shapefiles as a list:

library(rgdal)
library(raster)
library(sf)
library(maptools)

NOAA_normal <- raster("C:/path/to/raster/noaa_normal.tif")

input_path <- "C:/path/to/Counties"
files <- list.files(dir, pattern="[.]shp$", full.names=TRUE)
allShapes <- lapply(files, readOGR)

But I still need to create a loop that goes through each individual shapefile and performs the zonal statistics. The loop below is one I tried, but it only gives me a single value in return whereas I want the mean value for each polygon.

for (i in 1:length(allShapes)){
  ex <- extract(NOAA_normal_crop, allShapes[[i]], fun=mean, na.rm=TRUE, df=TRUE)
}

EDIT: I have also tried using lapply again, but this isn't working either.

lapply(allShapes, extract(NOAA_normal_crop, allShapes, fun=mean, na.rm=TRUE, df=TRUE))
# Error in round(y) : non-numeric argument to mathematical function

Thanks in advance for any advice you might have.

回答1:

I believe your loop is correct. However it is not a list or array, so every iteration the value is being overwritten. You should try: ex = vector() for (i in 1:length(allShapes)){ ex[i] <- extract(NOAA_normal_crop, allShapes[[i]], fun=mean, na.rm=TRUE, df=TRUE) }

来源：https://stackoverflow.com/questions/58187021/creating-r-loop-to-read-in-shapefiles-from-a-directory-and-perform-zonal-statist