在R中找到多边形的外边缘

Question

我有一个经度和纬度点的数据框，绘制时如下所示： 我想定义一个单独的数据框，其中包含所有外部左边缘。我在这里强调了几点： 。

我尝试使用 st_convex_hull() 函数以及 getBorders() （制图包的一部分），但这给了我整个外边缘，而不仅仅是我想要的左边缘。

Answer 1

这个解决方案使用最北点和最南点来用

lwgeom

分割边界。需要手动选择所需的“边缘”（三行中的两行）。我用

meuse

作为例子。使用

st_concave_hull(ratio=0.1)

获得边界，并在其上测试点。根据凹度，可能需要进行调整。

library(tidyverse)
library(sf)
#> Linking to GEOS 3.11.2, GDAL 3.7.2, PROJ 9.3.0; sf_use_s2() is TRUE
library(lwgeom) # for lwgeom::st_split
#> Warning: le package 'lwgeom' a été compilé avec la version R 4.3.3
#> Linking to liblwgeom 3.0.0beta1 r16016, GEOS 3.11.2, PROJ 9.3.1
#> Warning in fun(libname, pkgname): PROJ versions differ: lwgeom has 9.3.1 sf has
#> 9.3.0
#> 
#> Attachement du package : 'lwgeom'
#> L'objet suivant est masqué depuis 'package:sf':
#> 
#>     st_perimeter
library(sp) # for meuse

data(meuse) 
df_sf = meuse %>% st_as_sf(coords= c("x","y"),remove= FALSE) %>% 
  mutate(ID= 1:n())

ggplot(data=df_sf)+geom_sf()

# Boundary
st_simplify(df_sf %>% summarise()  %>% st_concave_hull(ratio=0.1) %>% st_cast(to ="LINESTRING")) %>% 
  st_as_sfc() %>% ggplot(data=.)+geom_sf()

# Getting the points
border_point =   df_sf %>% summarise()  %>% st_concave_hull(ratio=0.1) %>% st_cast(to ="POINT") %>% 
  st_join(df_sf, .predicate = st_equals) 

# Choosing start/end point. 
MinMax =    bind_rows(border_point %>% slice_max(y), 
                      border_point %>% slice_min(y))

# Splitting along those points
boundary_split = 
lwgeom::st_split(df_sf %>% summarise()  %>% st_concave_hull(ratio=0.1) %>% st_cast(to ="LINESTRING"),
                 MinMax ) %>% 
  st_collection_extract(., "LINESTRING") %>% 
  mutate(id_line = 1:n())

# Three lines are generated, since the start/end of the linestring acts as a splitting point as well
# Plotting to identify the releveants id_line (left shore), 
boundary_split %>% 
  ggplot(data=.)+geom_sf( aes(col = id_line %>% as.factor()))

# manually selecting 
left_boundary = boundary_split %>% filter( id_line %in% c(1,2)) 

left_points =  st_join(df_sf,left_boundary  ) %>%  
  # Removing unjoinned point (NA), there is certainly a better way
  filter( id_line %in% c(1,2)) 

# Ploting elements

ggplot()+
  geom_sf(data=df_sf %>% summarise()  %>% st_concave_hull(ratio=0.1),fill=NA)+
  geom_sf(data=df_sf,col ="grey")+
  geom_sf(data =MinMax, col ="green", size =5)+
  geom_sf(data = boundary_split,aes(col = id_line %>% as.factor()) )+
  geom_sf(data = left_points, col ="red")

^{创建于 2024-04-16，使用 reprex v2.1.0}