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r - 提取简单特征数据框中点的坐标

问题描述

我有 2 条线，我需要提取这 2 条线相交的坐标。我正在尝试使用 R 中的 sf 包来做到这一点。我可以做交叉点，我可以绘制它们，但我无法提取坐标。当每条线只有 1 个相交点时，它确实有效，但是当有多个（并且在实际数据中有很多）时，相交点的坐标存储在每条线的单个字段中。

inters_pt 看起来像：类几何 A c(3, 3, 3.2, 4.2) B c(1.5, 1.5) A c(2, 4)

我收到消息：

st_coordinates.sfc(st_geometry(x)) 中的错误：未针对 sfc_GEOMETRY 类的对象实现

我想提取每个交叉点的坐标并将属性保留在“类”列中。

输出应如下所示： 几何类 A (3, 3.2) A (3, 4.2) B (1.5, 1.5) A (2, 4)

虽然最初我认为它看起来像这样：几何类 A (3, 3) A (3.2, 4.2)

我错了。

搜索让我来到这里： https ://github.com/r-spatial/sf/issues/114

但是那里的解决方案要么掉点（st_cast（mp，“POINT”），失去属性'class'或抛出以下错误：

vapply(lst, class, rep(NA_character_, 3)) 中的错误：值的长度必须为 3，但 FUN(X[ 1 ]) 结果的长度为 2

#First create my data, I create points, and convert them into SpatialLines
lines1.df <- data.frame(
  x = c(1,2,3,3,3,3,1,2,3),
  y = c(1,2,2,3,4,5,4,4,4),
  id = c(rep("A",6), rep("B",3))
) 

#with Kyle Walker's functions I convert the points to lines
#https://rpubs.com/walkerke/points_to_line 

library(sp)
library(maptools)

points_to_line <- function(data, long, lat, id_field = NULL, sort_field = NULL) {

  # Convert to SpatialPointsDataFrame
  coordinates(data) <- c(long, lat)

  # If there is a sort field...
  if (!is.null(sort_field)) {
    if (!is.null(id_field)) {
      data <- data[order(data[[id_field]], data[[sort_field]]), ]
    } else {
      data <- data[order(data[[sort_field]]), ]
    }
  }

  # If there is only one path...
  if (is.null(id_field)) {

    lines <- SpatialLines(list(Lines(list(Line(data)), "id")))

    return(lines)

    # Now, if we have multiple lines...
  } else if (!is.null(id_field)) {  

    # Split into a list by ID field
    paths <- sp::split(data, data[[id_field]])

    sp_lines <- SpatialLines(list(Lines(list(Line(paths[[1]])), "line1")))

    # I like for loops, what can I say...
    for (p in 2:length(paths)) {
      id <- paste0("line", as.character(p))
      l <- SpatialLines(list(Lines(list(Line(paths[[p]])), id)))
      sp_lines <- spRbind(sp_lines, l)
    }

    return(sp_lines)
  }
}


lines1 <- points_to_line(data = lines1.df, 
                        long = "x", 
                        lat = "y", 
                        id_field = "id")

proj4string(lines1) <- CRS("+proj=utm +zone=32 +datum=WGS84")

library(sf)

lines1_sf <- st_as_sf(lines1,
                         crs = "+proj=utm +zone=32 +datum=WGS84")

lines2.df <- data.frame(
  x = c(2,2,2.5,3.5,3.5,2.5,1,2),
  y = c(3,4.2,4.2,4.2,3.2,3.2,2,1),
  id = c(rep("A",6), rep("B",2))
) 

lines2 <- points_to_line(data = lines2.df, 
                            long = "x", 
                            lat = "y", 
                            id_field = "id")

proj4string(lines2) <- CRS("+proj=utm +zone=32 +datum=WGS84")

lines2_sf <- st_as_sf(lines2,
                       crs = "+proj=utm +zone=32 +datum=WGS84")
#add attributes
lines2_sf$class <- c("A", "B")

#plot both lines
library(ggplot2)
ggplot(lines1_sf) +
  geom_sf(aes(color="red")) +
  geom_sf(data=lines2_sf) +
  coord_sf(crs = "+proj=utm +zone=32 +datum=WGS84")  +
  theme_bw()

#find intersecting points
#intersect 
inters_pt <- st_intersection(lines2_sf, lines1_sf)

#plot shows the intersecting points
ggplot(lines1_sf) +
  geom_sf(aes(color="red")) +
  geom_sf(data=lines2_sf) +
  geom_sf(data=inters_pt) +
  coord_sf(crs = "+proj=utm +zone=32 +datum=WGS84")  +
  theme_bw()

#extract the coordinates from the geometry column
st_coordinates(inters_pt) #<-- gives error

st_cast(inters_pt, "POINT") #removes points

y = as(inters_pt, "Spatial") #loses the 'class' attribute
SpatialPoints(y)

st_cast(st_sfc(inters_pt), "POINT", group_or_split = FALSE) #error

标签： rgispointsfmultipoint

解决方案

首先，值得使用 sf 从点创建线，这减少了所需的代码量：

library(sf)
library(dplyr)

#First create my data, I create points, and convert them into SpatialLines
lines1.df <- data.frame(
  x = c(1,2,3,3,3,3,1,2,3),
  y = c(1,2,2,3,4,5,4,4,4),
  id = c(rep("A",6), rep("B",3))
) 

lines2.df <- data.frame(
  x = c(2,2,2.5,3.5,3.5,2.5,1,2),
  y = c(3,4.2,4.2,4.2,3.2,3.2,2,1),
  id = c(rep("A",6), rep("B",2))
) 

points_to_line <- function(data, group = "id"){
  data <- data %>% 
    group_by_at(group) %>%
    summarise(do_union = FALSE) %>%
    st_cast("LINESTRING") %>%
    ungroup %>%
    select(-do_union)
}

crs <- "+proj=utm +zone=32 +datum=WGS84"
coords <- c("x", "y")
lines1_sf <- st_as_sf(lines1.df, coords = coords, crs = crs) %>%
  points_to_line()
lines2_sf <- st_as_sf(lines2.df, coords = coords, crs = crs) %>%
  points_to_line()

#add attributes
lines2_sf$class <- c("A", "B")

#find intersecting points
#intersect 
inters_pt <- st_intersection(lines1_sf, lines2_sf)

在这里，您需要先转换为 MULTIPOINT 然后再转换为 POINT

#extract the coordinates from the geometry column
inters_pt <- inters_pt %>%
  st_cast("MULTIPOINT") %>%
  st_cast("POINT")

> st_coordinates(inters_pt) 
#    X   Y
#1 3.0 3.2
#2 3.0 4.2
#3 2.0 4.0
#4 1.5 1.5

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