计算R中的加权多边形质心 [英] Calculating weighted polygon centroids in R
问题描述
我需要根据单独的人口网格数据集计算一组空间区域的质心.感谢您为下面的示例提供指导.
I need to calculate the centroids of a set of spatial zones based on a separate population grid dataset. Grateful for a steer on how to achieve this for the example below.
谢谢.
require(raster)
require(spdep)
require(maptools)
dat <- raster(volcano) # simulated population data
polys <- readShapePoly(system.file("etc/shapes/columbus.shp",package="spdep")[1])
# set consistent coordinate ref. systems and bounding boxes
proj4string(dat) <- proj4string(polys) <- CRS("+proj=longlat +datum=NAD27")
extent(dat) <- extent(polys)
# illustration plot
plot(dat, asp = TRUE)
plot(polys, add = TRUE)
推荐答案
三个步骤:
首先,找到每个多边形中的所有单元格,并返回一个包含单元格编号和值的2列矩阵的列表:
First, find all the cells in each polygon, return a list of 2-column matrices with the cell number and the value:
require(plyr) # for llply, laply in a bit...
cell_value = extract(dat, polys,cellnumbers=TRUE)
head(cell_value[[1]])
cell value
[1,] 31 108
[2,] 32 108
[3,] 33 110
[4,] 92 110
[5,] 93 110
[6,] 94 111
第二,转到相似矩阵的列表,但添加x和y坐标:
Second, turn into a list of similar matrices but add the x and y coords:
cell_value_xy = llply(cell_value, function(x)cbind(x,xyFromCell(dat,x[,"cell"])))
head(cell_value_xy[[1]])
cell value x y
[1,] 31 108 8.581164 14.71973
[2,] 32 108 8.669893 14.71973
[3,] 33 110 8.758623 14.71973
[4,] 92 110 8.581164 14.67428
[5,] 93 110 8.669893 14.67428
[6,] 94 111 8.758623 14.67428
第三,计算加权平均坐标.这忽略了任何边缘效果,并假设所有网格单元的大小相同:
Third, compute the weighted mean coordinate. This neglects any edge effects and assumes all grid cells are the same size:
centr = laply(cell_value_xy, function(m){c(weighted.mean(m[,3],m[,2]), weighted.mean(m[,4],m[,2]))})
head(centr)
1 2
[1,] 8.816277 14.35309
[2,] 8.327463 14.02354
[3,] 8.993655 13.82518
[4,] 8.467312 13.71929
[5,] 9.011808 13.28719
[6,] 9.745000 13.47444
现在,centr
是一个2列矩阵.在您的示例中,该示例非常接近coordinates(polys)
,因此我将创建一个人为设计的示例,并使用一些极端的权重以确保其按预期工作.
Now centr
is a 2-column matrix. In your example its very close to coordinates(polys)
so I'd make a contrived example with some extreme weights to make sure its working as expected.
这篇关于计算R中的加权多边形质心的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!