r - 如何从一组段(spatstat 中的 psp)中获取 SpatialPolygons(SP 类)？

Question

我有一组随机片段在 window 中绘制一种镶嵌(三角形、矩形......) (在 spatstat R 中)。我需要将它转换成一组多边形 (SpatialPolygons) 来计算一些指标(如面积、形状指标......)。

这看起来很简单，但我找不到如何去做......

这是来自 Carl Witthoft 的一些代码生成自拦截段的随机模式:

ranpoly <- function(numsegs=10,plotit=TRUE) {

require(spatstat)
# temp fix: put the first seg into segset. Later make it a constrained random.
segset<-psp(c(0,1,1,0,.25),c(0,0,1,1,0),c(1,1,0,0,1),c(0,1,1,0,.75),owin(c(0,1),c(0,1)) ) #frame the frame
for (jj in 1: numsegs) {
# randomly select a segment to start from, a point on the seg, the slope,and direction
# later... watch for slopes that immediately exit the frame
endx <-sample(c(-0.2,1.2),1)  #force 'x1' outside the frame
# watch that sample() gotcha
if(segset$n<=5) sampset <- c(5,5) else sampset<-5:segset$n
startseg<-sample(sampset,1) #don't select a frame segment
# this is slope of segment to be constructed
slope <- tan(runif(1)*2*pi-pi) # range +/- Inf 
# get length of selected segment
seglen<-lengths.psp(segset)[startseg]
startcut <- runif(1) 
# grab the coords of starting point (similar triangles)
startx<- segset$ends$x0[startseg] + (segset$ends$x1[startseg]-segset$ends$x0[startseg])*startcut #seglen
starty<- segset$ends$y0[startseg] + (segset$ends$y1[startseg]-segset$ends$y0[startseg])*startcut #seglen
# make a psp object with that startpoint and slope; will adjust it after finding intersections
endy <- starty + slope*(endx-startx)
newpsp<-psp(startx,starty,endx,endy,segset$window,check=FALSE)
# don't calc crossing for current element of segset
hits <- crossing.psp(segset[-startseg],newpsp)
segdist <- dist(cbind(c(startx,hits$x),c(starty,hits$y)))
# dig back to get the crosspoint desired -- have to get matrixlike object out of class "dist" object
# And, as.matrix puts a zero in location 1,1 kill that row.
cutx <- hits$x[ which.min( as.matrix(segdist)[-1,1] )]
cuty <- hits$y[which.min(as.matrix(segdist)[-1,1] )]
segset <- superimpose(segset,psp(startx,starty,cutx,cuty,segset$window))

} #end jj loop
if(plotit) plot(segset,col=rainbow(numsegs))
return(invisible(segset))
}

segset=ranpoly()

segset 是我需要创建 SpatialPolygons 对象的 psp 对象。

Answer 1

谷歌搜索 spatstat as spatialPolygons 将我带到这个 first hit ，这是 spatstat 中专门用于处理 shapefile 的小插图。它在如何将 sp 类转换为 spatstat 对象上花费了大量时间。您可能对第 3.2.5 节:SpatialPolygons 类的对象 和第 3.2.6 节:SpatialPolygonsDataFrame 类的对象最感兴趣。