在给定点的多边形内创建线 [英] Create lines within polygons at given points

问题描述

给定一个空间多边形

library(sf)

df <- data.frame(
  lon = c(119.4, 119.4, 119.4, 119.5, 119.5),
  lat = c(-5.192, -5.192, -5.167, -5.167, -5.191)
)

polygon <- df %>%
  st_as_sf(coords = c("lon", "lat"), crs = 4326) %>%
  summarise(geometry = st_combine(geometry)) %>%
  st_cast("POLYGON")

里面有几个点

df2 <- data.frame(
  lon = c(119.45, 119.49, 119.47),
  lat = c(-5.172, -5.190, -5.183)
)

points <- df2 %>%
  st_as_sf(coords = c("lon", "lat"), crs = 4326) %>%
  summarise(geometry = st_combine(geometry)) %>%
  st_cast("MULTIPOINT")

如何通过每个点绘制一条从多边形的一端到另一端的水平线？也就是说，在多边形内绘制三条直线，每条直线穿过其中一个点。我发现了几个绘制连接点或使用点作为起点和终点的线条的例子，但在我的例子中，这些点只是标记所需的水平线的y值。

提前谢谢！

推荐答案

此方法只是在点上循环，并使用POLYGON的x坐标和POINT的y坐标创建一个新的LINESTRING对象：

library(sf)
library(dplyr)

df <- data.frame(
  lon = c(119.4, 119.4, 119.4, 119.5, 119.5),
  lat = c(-5.192,-5.192,-5.167,-5.167,-5.191)
)

polygon <- df %>%
  st_as_sf(coords = c("lon", "lat"), crs = 4326) %>%
  summarise(geometry = st_combine(geometry)) %>%
  st_cast("POLYGON")

plot(polygon)

df2 <- data.frame(lon = c(119.45, 119.49, 119.47),
                  lat = c(-5.172,-5.190,-5.183))

points <- df2 %>%
  st_as_sf(coords = c("lon", "lat"), crs = 4326) %>%
  summarise(geometry = st_combine(geometry)) %>%
  st_cast("MULTIPOINT")

plot(points, add = TRUE, col = "red")


# Solution via a loop

xminmax <- c(min(df$lon), max(df$lon))

# Iterate and create lines
for (i in 1:3) {
  l <- st_linestring(matrix(
    c(xminmax[1],
      df2[i, "lat"],
      xminmax[2],
      df2[i, "lat"]),
    nrow = 2,
    byrow = TRUE
  ))
  # Create final object line
  if (i == 1) {
    line <- l
  } else {
    line <- c(line, l)
  }
}

# Result is line MULTILINESTRING object


plot(line, col = "green", add = TRUE)

^{由reprex package(v1.0.0)}在2021-02-15创建

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