用 st_buffer 围绕一个地理点

Question

我想使用 sf 包在都柏林机场周围绘制一个 110 海里（海里）的圆。（稍后我将通过 st_intersect 与来自 ADS-B 的飞行位置报告相交。）

我为 NM 定义了一个新单位如下：

library(units)
library(tidyverse)
library(sf)
NM <- make_unit("NM")
install_conversion_constant("NM", "km", 1.852)

然后定义都柏林机场坐标：

# DUB/EIDW location, see 
# https://skyvector.com/airport/EIDW/Dublin-Airport
# Coordinates:
#   N53°25.28' / W6°16.20' (Degrees Decimal Minutes (DDM) format)
#   (-6.27, 53.421333) (lon/lat Decimal Degrees (DD))
# Elevation: 242.0 feet (MSL)
dub_lon <- -6.27
dub_lat <- 53.421333
dub_elv <- set_units(242.0, ft)

dub <- st_point( x = c(dub_lon, dub_lat, dub_elv), dim = "XYZ")
dub <- dub %>% st_sfc(crs = 4326)

因此定义了机场周围圆的半径（以米为单位）：

r110 <- set_units(110, NM) %>% set_units(km)

现在，当我尝试 st_buffer 时，一切都不起作用：

> r110 <- set_units(110, NM) %>% set_units(km)
Error: cannot convert km into Â°
In addition: Warning message:
In st_buffer.sfc(dub, dist = r110) :
  st_buffer does not correctly buffer longitude/latitude data, dist needs to be in decimal degrees.

如果我尝试传递一个数值（203.72，这些是公里）作为距离，至少我只会收到警告：

> dub110 <- st_buffer(dub, dist = 203.72)
Warning message:
In st_buffer.sfc(dub, dist = 203.72) :
  st_buffer does not correctly buffer longitude/latitude data, dist needs to be in decimal degrees.

但是绘制它显示了一个太大的圆圈

library(mapview)
mapview(dub110)

我应该在 st_buffer 中输入 dist 的单位是什么？我阅读了文档，但并没有真正找到该怎么做...

任何hints/helps真的很感激！

Answer 1

如 Phil 所述，您需要将坐标转换为米/'distance' 投影，而不是基于度数的投影。

我不熟悉 sf，但是 sp...

library(sp) 
dub_transformed <- spTransform(dub,CRS("+init=epsg:29902"))

...在您运行之前，缓冲区命令应该足够了。然后您可能希望将缓冲对象转换回 epsg:4326 以进行 plotting/additional 处理。例如

dub110 <- spTransform(dub110,CRS("+init=epsg:4326"))

Answer 2

如果您愿意，这里有一个纯粹的 sf 答案，但@Jul 的答案非常有用。

设置为您的示例：

library(units)
library(tidyverse)
library(sf)
NM <- make_unit("NM")
install_conversion_constant("NM", "km", 1.852)

# DUB/EIDW location, see 
# https://skyvector.com/airport/EIDW/Dublin-Airport
# Coordinates:
#   N53°25.28' / W6°16.20' (Degrees Decimal Minutes (DDM) format)
#   (-6.27, 53.421333) (lon/lat Decimal Degrees (DD))
# Elevation: 242.0 feet (MSL)
dub_lon <- -6.27
dub_lat <- 53.421333
dub_elv <- set_units(242.0, ft)

dub <- st_point(x = c(dub_lon, dub_lat, dub_elv), dim = "XYZ")
dub <- dub %>% st_sfc(crs = 4326)

然后将坐标转换为Irish Grid:

dub = st_transform(dub, 29902)

在这一点附近以米为单位创建缓冲区：

dub_buffer = st_buffer(dub, 110000)

绘制结果：

plot(dub_buffer)
plot(dub, add = TRUE)

Answer 3

感谢Phil and Jul 初始问题的完整解决方案如下：

library(units)
library(tidyverse)
library(sf)
library(mapview)
library(units)

# define nautical miles (as per ICAO notation)
NM <- make_unit("NM")
install_conversion_constant("NM", "km", 1.852)

# DUB/EIDW location, see
# https://skyvector.com/airport/EIDW/Dublin-Airport
# Coordinates:
#   N53°25.28' / W6°16.20' (Degrees Decimal Minutes (DDM) format)
#   (-6.27, 53.421333) (lon/lat Decimal Degrees (DD))
# Elevation: 242.0 feet (MSL)
dub_lon <- -6.27
dub_lat <- 53.421333
dub_elv <- set_units(242.0, ft)
dub <- st_point(x = c(dub_lon, dub_lat, dub_elv), dim = "XYZ")
dub <- dub %>% st_sfc(crs = 4326)

# define radious of interest, i.e. 110 NM
r110 <- set_units(110, NM) %>% set_units(km) %>% set_units(m)

# change to Irish grid, which uses meters
dub <- st_transform(dub, 29902)
dub_buffer <-  st_buffer(dub, r110)

# eventually convert back to WSG84 if needed for other purposes
dub <- st_transform(dub, 4326)
dub_buffer <- st_transform(dub_buffer, 4326)
mapview(dub_buffer)

用 st_buffer 围绕一个地理点

circle around a geographic point with st_buffer

r

sf