R数据点周长缓冲区

问题描述：

我需要在带有x和y坐标（图上的灰点）的data points集上创建一个缓冲区。不幸的是，我没有从中创建缓冲区的点的周边边界。我试图用chull函数来计算周长，但是它不能正常工作（橙色区域）。我可以使用max/min函数来计算边界点，通过某个步骤（比如说10米，红点）来计算数据，并尝试从这些点计算缓冲区。有人知道更正确和干净的方法来计算点集的缓冲区。R数据点周长缓冲区

是红线，你正在寻找的结果？ – Thierry

我想检索所有落在由红线标识的多边形内的点。 – Vova

'chull'工作正常。你只是不想让你的船体“凸出”。 –

答

非常感谢您的建议和意见。确实，这是我的错，省略了alphahull包。

在识别与ashape的边界后，我创建了一个缓冲区多边形，并标识了位于缓冲区内部和外部的数据。挑战是从ashap正确提取多边形，但解决方案RPubs安全我。您还可以看到图形化的例子here.

最佳

## load 
library(ggplot2); library(alphahull); 
library(igraph); library(rgeos) 
## Load the data 
data.df<-read.csv("Data/Cencus/Lyford_meta.csv",sep=",",header=TRUE) 

#Remove the duplicates in the data to do the chull calculation 
data <- data.df[!duplicated(paste(data.df$xsite, data.df$ysite, sep ="_")), c("xsite","ysite") ] 

#calculate the chull with alpha 20 
data.chull <- ashape(data, alpha = 20) 


## Below is the code to extract polygon from the ashape chull function 
## credit to: http://rpubs.com/geospacedman/alphasimple 
order.chull <- graph.edgelist(cbind(as.character(data.chull$edges[, "ind1"]), as.character(data.chull$edges[,"ind2"])), directed = FALSE) 
cutg <- order.chull - E(order.chull)[1] 
ends <- names(which(degree(cutg) == 1)) 
path <- get.shortest.paths(cutg, ends[1], ends[2])[[1]] 
pathX <- as.numeric(V(order.chull)[unlist(path[[1]])]$name) 
pathX = c(pathX, pathX[1]) 
data.chull <- as.data.frame(data.chull$x[pathX, ]) 


## Create a spatial object from the polygon and apply a buffer to 
## Then extract the data to the dataframe. 
data.chull.poly <- SpatialPolygons(list(Polygons(list(Polygon(as.matrix(data.chull))),"s1"))) 
data.chull.poly.buff <- gBuffer(data.chull.poly, width = -10) 
data.buffer <- fortify(data.chull.poly.buff)[c("long","lat")] 

## Identidfy the data that are inside the buffer polygon 
data$posit <- "Outside" 
data$posit[point.in.polygon(data$x,data$y,data.buffer$long,data.buffer$lat) %in% c(1,2,3)] <- "Inside" 


## Plot the results 
ggplot()+ 
    theme_bw()+xlab("X coordinates (m)")+ylab("Y coordinates (m)") + 
    geom_point(data = data, aes(xsite, ysite, color = posit))+ 
    geom_polygon(data = data.chull, aes(V1, V2), color = "black", alpha = 0)+ 
    geom_polygon(data = data.buffer, aes(long, lat), color = "blue", alpha = 0)

答

你可以做点周围镶嵌。边界上的点将具有更大的多边形。

library(deldir) 
library(ggplot2) 
triang <- deldir(data$x, data$y) 
border <- triang$summary 
border$Selected <- border$dir.area > 260 
ggplot(border[order(border$Selected), ], aes(x = x, y = y, colour = Selected)) + geom_point()

R数据点周长缓冲区

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