r - 按固定距离缩小 SF 多边形

Question

我正在尝试按固定距离缩小 sf 多边形。
以这个简单的多边形为例:

coords <- list(rbind(c(0,0), c(2,0), c(2,2), c(1,2), c(1,1), c(0,1), c(0,0)))
p <- sf::st_polygon(x = coords)
ggplot() + geom_sf(data = p,  fill = "white", color = "black")

我想制作一个形状相同的多边形，但边缘都与原始多边形相距 0.1 个单位。这是目标:

target.coords <- list(rbind(c(0.1,0.1), c(1.9,0.1), c(1.9,1.9), c(1.1,1.9), c(1.1,0.9), c(0.1,0.9), c(0.1,0.1)))
target <- sf::st_polygon(x = target.coords)
ggplot() + 
  geom_sf(data = p,  fill = "white", color = "black") +
  geom_sf(data = target,  fill = "green", color = "dark green")

我以为 sf::st_buffer()会让我到那里，但外角不太匹配。

p1 <- sf::st_buffer(x = p, dist = -0.1)
ggplot() +
  geom_sf(data = p,  fill = "white", color = "black") + 
  geom_sf(data = p1, fill = "red",   color = "dark red")

是否有替代设置 sf::st_buffer()那可以让我实现目标多边形吗？或者是否有替代功能可以实现这一目标？偏移量需要固定距离，而不是将多边形缩小相对量(例如 90%)。

Answer 1

如果问题只是与那个角落有关，我认为您可以更改 st_buffer 的默认行为使用参数 joinStyle和 mitreLimit .例如:

# packages
library(sf)
#> Linking to GEOS 3.9.0, GDAL 3.2.1, PROJ 7.2.1
library(ggplot2)

# data
coords <- list(rbind(c(0,0), c(2,0), c(2,2), c(1,2), c(1,1), c(0,1), c(0,0)))
p <- sf::st_polygon(x = coords)
p1 <- st_buffer(x = p, dist = -0.1, joinStyle  = "MITRE", mitreLimit = 2)

# plot
ggplot() +
  geom_sf(data = p,  fill = "white", color = "black") + 
  geom_sf(data = p1, fill = "red",   color = "dark red")

# Check 
target.coords <- list(rbind(c(0.1,0.1), c(1.9,0.1), c(1.9,1.9), c(1.1,1.9), c(1.1,0.9), c(0.1,0.9), c(0.1,0.1)))
(target <- sf::st_polygon(x = target.coords))
#> POLYGON ((0.1 0.1, 1.9 0.1, 1.9 1.9, 1.1 1.9, 1.1 0.9, 0.1 0.9, 0.1 0.1))
st_equals(p1, target)
#> Sparse geometry binary predicate list of length 1, where the predicate was `equals'
#>  1: 1

创建于 2021-06-17 由 reprex package (v2.0.0)
另见 here有关 st_buffer() 的更多示例.