光栅::光栅到轮廓；轮廓线不连续

我正在尝试使用 R 中的 raster 包从光栅对象中提取轮廓线.

I am trying to extract contour lines from a raster object using the raster package in R.

rasterToContour 似乎运行良好，绘图也很好，但在调查时发现等高线被分解成不规则的部分.来自 ?rasterToContour

rasterToContour appears to work well and plots nicely, but when investigated it appears the contour lines are broken up into irregular segments. Example data from ?rasterToContour

library(raster)

f <- system.file("external/test.grd", package="raster")
r <- raster(f)
x <- rasterToContour(r)
class(x)
plot(r)
plot(x, add=TRUE)

我正在尝试提取栅格中示例站点的轮廓线.因此，我们随机选择一个站点，提取其高程，然后再次运行 rasterToContour()，指定等高线 level 的高程.

I am trying to extract the contour line of a sample site in the raster. So, we choose a random site, extract its elevation, and run rasterToContour() again, specifying the elevation for the contour line level.

# our sample site - a random cell chosen on the raster
xyFromCell(r, 5000) %>%
  SpatialPoints(proj4string = crs(r)) %>%
  {. ->> site_sp} %>%
  st_as_sf %>%
  {. ->> site_sf}

# find elevation of sample site, and extract contour lines
extract(r, site_sf) %>%
  {. ->> site_elevation}

# extract contour lines
r %>%
  rasterToContour(levels = site_elevation) %>%
  {. ->> contours_sp} %>%
  st_as_sf %>%
  {. ->> contours_sf}

# plot the site and new contour lines (approx elevation 326)
plot(r)
plot(contours_sf, add = TRUE)
plot(site_sf, add = TRUE)

# plot the contour lines and sample site - using sf and ggplot
ggplot()+
  geom_sf(data = contours_sf)+
  geom_sf(data = site_sf, color = 'red')

然后我们使用 st_intersects 来查找与站点相交的线(缓冲区宽度为 100 以确保它与线接触).但是，这将返回所有等高线.

Then we use st_intersects to find the lines that intersect the site (with a buffer width of 100 to ensure it touches the line). But, this returns all of the contour lines.

contours_sf %>%
  filter(
    st_intersects(., site_sf %>% st_buffer(100), sparse = FALSE)[1,]
  ) %>%
  ggplot()+
  geom_sf()

我假设所有行都被返回，因为它们看起来是一个 MULTILINESTRING sf 对象.

I assume all lines are returned because they appear to be a single MULTILINESTRING sf object.

contours_sf

# Simple feature collection with 1 feature and 1 field
# geometry type:  MULTILINESTRING
# dimension:      XY
# bbox:           xmin: 178923.1 ymin: 329720 xmax: 181460 ymax: 333412.3
# CRS:            +proj=sterea +lat_0=52.1561605555556 +lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 +datum=WGS84 +units=m +no_defs
# level                       geometry
# C_1 326.849822998047 MULTILINESTRING ((179619.3 ...

因此，我使用 ngeo::st_segments() 将 contours_sf MULTILINESTRING 拆分为单独的行(我找不到任何 sf 方法来做到这一点，但我愿意使用替代方法，特别是如果这是问题所在.

So, I have split the contours_sf MULTILINESTRING into individual lines using ngeo::st_segments() (I couldn't find any sf way to do this, but am open to using alternative methods, especially if this is the problem).

没想到这会返回 394 个特征；从看图来看，我预计大约有 15 行.

Unexpectedly this returns 394 features; from looking at the figure I would expect approximately 15 separate lines.

contours_sf %>%
  nngeo::st_segments()

# Simple feature collection with 394 features and 1 field
# geometry type:  LINESTRING
# dimension:      XY
# bbox:           xmin: 178923.1 ymin: 329720 xmax: 181460 ymax: 333412.3
# CRS:            +proj=sterea +lat_0=52.1561605555556 +lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 +datum=WGS84 +units=m +no_defs
# First 10 features:
#   level                         result
# 1  326.849822998047 LINESTRING (179619.3 329739...
# 2  326.849822998047 LINESTRING (179580 329720.4...
# 3  326.849822998047 LINESTRING (179540 329720, ...
# 4  326.849822998047 LINESTRING (179500 329735.8...
# 5  326.849822998047 LINESTRING (179495.3 329740...
# 6  326.849822998047 LINESTRING (179460 329764, ...
# 7  326.849822998047 LINESTRING (179442.6 329780...
# 8  326.849822998047 LINESTRING (179420 329810, ...
# 9  326.849822998047 LINESTRING (179410.2 329820...
# 10 326.849822998047 LINESTRING (179380 329847.3...

然后，当我们过滤以仅保留与站点相交的线(缓冲区宽度为 100)时，仅返回预期轮廓线的一小部分(线的红色部分，我假设反映了 100 缓冲区宽度).

Then, when we filter to keep only the lines which intersect the site (with a buffer width of 100), only a small section of the expected contour line is returned (red section of line, I assume reflective of the 100 buffer width).

contours_sf %>%
  nngeo::st_segments() %>%
  filter(
    # this syntax used as recommended by this answer https://stackoverflow.com/a/57025700/13478749
    st_intersects(., site_sf %>% st_buffer(100), sparse = FALSE)
  ) %>%

  ggplot()+
  geom_sf(colour = 'red', size = 3)+
  geom_sf(data = contours_sf)+
  geom_sf(data = site_sf, colour = 'cyan')+
  geom_sf(data = site_sf %>% st_buffer(100), colour = 'cyan', fill = NA)

任何人都对以下几点有想法:

Anyone got ideas for the following points:

1. 解释为什么等高线断了"
2. 提供一种将碎片连接"在一起的有效方法
3. nngeo::st_segments() 的替代方案，如果这实际上是 394 行而不是 ~15 行的来源

1. Explain why the contour lines are 'broken'
2. Provide an efficient method for 'joining' the broken pieces together
3. An alternative to nngeo::st_segments(), if this is in fact the source of the 394 lines not ~15