st_intersection 具有 POLYGON 边界的 LINESTRING，同时保留 r / sf 中点的顺序

Question

我需要将 LINESTRING 与边界 POLYGON 要素相交，同时保留生成的 POINT 要素的顺序。背景是我需要弄清楚汽车在 entering/leaving 特定国家/地区的过境点，但过境点的顺序很重要。

我已经实施了以下方法：

# setup test data
  poly <- 
    list(matrix(c(0,0,10,0,10,10,0,10,0,0),ncol=2, byrow=TRUE)) %>% 
    sf::st_polygon() %>% 
    sf::st_sfc() %>% 
    sf::st_sf()

  line1 <- matrix(c(-1, 10, 5, -1),ncol=2, byrow=TRUE) %>% 
    sf::st_linestring() %>% 
    sf::st_sfc() %>% 
    sf::st_sf()

  # reverse of line 1
  line2 <- matrix(c(5, -1, -1, 10), ncol=2, byrow=TRUE) %>% 
    sf::st_linestring() %>% 
    sf::st_sfc() %>% 
    sf::st_sf()

  # preview
  leaflet::leaflet() %>% 
    leaflet::addPolygons(data = poly) %>% 
    leaflet::addPolylines(data = line1) %>%
    leaflet::addPolylines(data = line2) %>% 
    leaflet::addTiles()


# do the intersection
  # cast to multilinestring because I just need the border crossing points
  ml <- sf::st_cast(poly, "MULTILINESTRING") 
  sf::st_intersection(ml, line1)
  sf::st_intersection(ml, line2)

但是，这种方法丢失了过境点的顺序。有人有更好的主意吗？

Answer 1

我找到了一个涉及 rgeos::gProject() 的答案。这个函数计算沿直线的点之间的距离，我可以用它来推导我的点的排序顺序：

# do the intersection
ml <- sf::st_cast(poly, "MULTILINESTRING") 

points1 <- sf::st_intersection(ml, line1) %>% 
  sf::st_cast("POINT")

points2 <- sf::st_intersection(ml, line2) %>% 
  sf::st_cast("POINT")

# Calculate sort order for the points
points1$order <- rgeos::gProject(sf::as_Spatial(line1), sf::as_Spatial(points1))
points2$order <- rgeos::gProject(sf::as_Spatial(line2), sf::as_Spatial(points2))

st_intersection 具有 POLYGON 边界的 LINESTRING，同时保留 r / sf 中点的顺序

st_intersection LINESTRING with borders of POLYGONs while preserving the order of points in r / sf

r

gis

spatial

sf