带 velox 的加权平均值

Question

我想根据一组多边形上的栅格计算人口加权平均测量值。出于性能原因，我更愿意使用 velox，但我不知道如何将权重合并到多边形平均中。下面是使用 raster.

演示加权平均的 MWE

library(raster)
library(sf)

rm(list = ls())

## Make matrix
dim <- c(5, 5)
set.seed(0)
data.mat <- matrix(runif(prod(dim), 0, 100), dim[1], dim[2])
extent <- c(0,1,0,1)
res <- 1/dim
vx <- velox(data.mat, extent, res, crs="")
rast <- vx$as.RasterLayer() # Save rast for comparison to raster::extract() and plotting

## Create sf polygon
pol <-
  st_sfc(st_polygon(list(cbind(
    c(.1, .4, .7, .1), c(.1, .8, .1, .1)
  ))))

## Weighted extract using raster
pol_sp <- as(pol, "Spatial")
wts <- raster::extract(rast, pol_sp, weights = T, normalizeWeights = T, cellnumbers = T, df = T)
weighted.mean(wts$layer, wts$weight) # Weighted average
# [1] 60.43645

Answer 1

下面的代码将 return 上面代码运行之后来自 velox 的加权平均值。对于大型多边形，我相信这可能比 raster::extract 快很多倍。这个答案的灵感来自 https://github.com/hunzikp/velox/issues/16.

## Weighted extract using velox
vx_get_weights <- function(rast, poly, normalizeWeights = T) {
  rast$cell <- 1:ncell(rast)
  brk_100 <- disaggregate(rast, fact = 10) 
  brk_100_vx <- velox(brk_100) 
  vx_raw_dt <- setDT(brk_100_vx$extract(poly, fun = NULL, df = TRUE))
  setnames(vx_raw_dt, c("poly_id", "x", "cell"))

  weights <- vx_raw_dt[, .(w = .N / 100), by = .(poly_id, cell, x)]
  if (normalizeWeights) {
    weights[, w := w / sum(w), by = poly_id]
  }
  setorder(weights, poly_id, cell)
  weights
}
weights <- vx_get_weights(rast, pol, normalizeWeights = T)
weighted.mean(rast[weights$cell], weights$w)
# [1] 60.43645

带 velox 的加权平均值

Weighted means with velox

r

raster

velox