Question

列表中的每个元素都包含一组空间坐标，我希望使用sf将其转换为多边形。每一组坐标按我想“连接点”的顺序排序，并且第一行和最后一行相同，以关闭多边形。每个列表元素都有一个唯一的标识符命名，我希望将其保留为sf输出中的属性。

我在这里修改了与科幻小说相关的答案中的代码：

Convert sequence of longitude and latitude to polygon via sf in R

但是我的情况有所不同，因为我有多组坐标（每个坐标都应生成一个单独的多边形），而该问题只有一组坐标（导致一个多边形）。

我的具体问题是如何使用sf生成一个sf多边形对象，该对象在单独的行中包含多个多边形，每个多边形都是使用列表元素之一中的坐标创建的。

在此先感谢您的任何建议或帮助。

标记

我的示例数据来自dput（），位于该问题的末尾，我的代码为：

points_df<-arrange(dat,SitePondGpsRep,DateTime_local) #sort on DateTime_local for proper sequence

points_df<-dplyr::select(points_df,SitePondGpsRep,Longitude,Latitude) #drop columns, for upcoming st_polygon call (requires numerics only)

points_ls<-split(points_df,points_df$SitePondGpsRep) #dataframe to list

points_ls<-lapply(points_ls, function(x) { x["SitePondGpsRep"] <- NULL; x }) #delete SitePondGpsRep column, it’s retained in list names

points_ls<-lapply(points_ls,function(x) {as.matrix(x)}) #convert to matrix for upcoming st_sf call

points_ls<-lapply(points_ls,function(x) {rbind(x,x[1,])}) #close poly, first and last point must be same

polys <- st_sf(st_sfc(st_polygon(points_ls)), crs = 4326) #create polys, but only one polygon is created when I expected three polygons

str(polys); glimpse(polys); plot(polys) #check output

样本数据：

dat <- structure(list(SitePondGpsRep = c("BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", "BURR-1-1-1", 
"BURR-1-1-1", "BURR-1-1-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", 
"BURR-1-3-1", "BURR-1-3-1", "BURR-1-3-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", 
"BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1", "BURR-1-4-1"
), DateTime_local = c("2018-05-30 10:49:04", "2018-05-30 10:49:05", 
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"2018-05-30 10:50:16"), Latitude = c(51.9851623569, 51.9851641171, 
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-177L))

Answer 1

与使用split和lapply相比，更简单的方法是利用sf的功能与dplyr工具（尤其是分组操作）一起良好地工作。我们可以：

使用coords的{{1}}参数为多边形的每个顶点创建点，
st_as_sf您的ID和group_by，将每个多边形的点合并为summarise，
使用MULTIPOINT转换为st_cast，在每个顶点之间画线。

POLYGON

由reprex package（v0.2.0）于2018-10-05创建。

Answer 2

从20191004开始在CRAN上的

library(sfheaders)可将data.frames转换为sf对象。

library(sf)
library(sfheaders)

sf <- sfheaders::sf_polygon(
  obj = dat
  , x = "Longitude"
  , y = "Latitude"
  , polygon_id = "SitePondGpsRep"
)

sf
# Simple feature collection with 3 features and 1 field
# geometry type:  POLYGON
# dimension:      XY
# bbox:           xmin: -105.077 ymin: 51.98512 xmax: -105.0761 ymax: 51.98542
# epsg (SRID):    NA
# proj4string:    
#   id                       geometry
# 1 BURR-1-1-1 POLYGON ((-105.0768 51.9851...
# 2 BURR-1-3-1 POLYGON ((-105.0768 51.9851...
# 3 BURR-1-4-1 POLYGON ((-105.0768 51.9851...

使用SF将空间坐标集转换为R中的多边形

2 个答案: