【发布时间】：2018-06-25 22:04:15
【问题描述】：
我有一个点集合，有一个 LINEID 和一个 ID_SEQ。 LINEID 确定唯一的 Line-IDS，而 ID_SEQ 确定 Line-ID 中点的顺序。
我想将点转换为线，按 ID_SEQ 排序并按 LINEID 分组。
使用sp 包，我能够达到预期的效果，但我想使用sf 包来做到这一点。我在这里错过了什么？
这里有一些虚拟数据，用sp-functions 说明了想要的结果，两次尝试用sf 做同样的事情，但是图表显示了不同的结果。
library(sp)
library(sf)
sfpoints <- {
structure(list(LINEID = c(4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L,
4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 10L, 10L,
10L), ID_SEQ = c(10L, 11L, 12L, 13L, 16L, 30L, 31L, 32L, 33L,
34L, 35L, 36L, 37L, 38L, 39L, 40L, 41L, 42L, 43L, 44L, 45L, 46L,
1L, 2L, 3L), geometry = structure(list(structure(c(15.423568,
47.06248), class = c("XY", "POINT", "sfg")), structure(c(15.423644,
47.062523), class = c("XY", "POINT", "sfg")), structure(c(15.423691,
47.062553), class = c("XY", "POINT", "sfg")), structure(c(15.423712,
47.06257), class = c("XY", "POINT", "sfg")), structure(c(15.423716,
47.062576), class = c("XY", "POINT", "sfg")), structure(c(15.423712,
47.062588), class = c("XY", "POINT", "sfg")), structure(c(15.423731,
47.062595), class = c("XY", "POINT", "sfg")), structure(c(15.423779,
47.062626), class = c("XY", "POINT", "sfg")), structure(c(15.423835,
47.062664), class = c("XY", "POINT", "sfg")), structure(c(15.423879,
47.062714), class = c("XY", "POINT", "sfg")), structure(c(15.423897,
47.062767), class = c("XY", "POINT", "sfg")), structure(c(15.423862,
47.062828), class = c("XY", "POINT", "sfg")), structure(c(15.423783,
47.062897), class = c("XY", "POINT", "sfg")), structure(c(15.423681,
47.062973), class = c("XY", "POINT", "sfg")), structure(c(15.423564,
47.06306), class = c("XY", "POINT", "sfg")), structure(c(15.423437,
47.063164), class = c("XY", "POINT", "sfg")), structure(c(15.42331,
47.06327), class = c("XY", "POINT", "sfg")), structure(c(15.423186,
47.063385), class = c("XY", "POINT", "sfg")), structure(c(15.423062,
47.063496), class = c("XY", "POINT", "sfg")), structure(c(15.422941,
47.063602), class = c("XY", "POINT", "sfg")), structure(c(15.422821,
47.063717), class = c("XY", "POINT", "sfg")), structure(c(15.422699,
47.063824), class = c("XY", "POINT", "sfg")), structure(c(15.422518,
47.061687), class = c("XY", "POINT", "sfg")), structure(c(15.422617,
47.06179), class = c("XY", "POINT", "sfg")), structure(c(15.422717,
47.061893), class = c("XY", "POINT", "sfg"))), class = c("sfc_POINT",
"sfc"), precision = 0, bbox = structure(c(15.422518, 47.061687,
15.423897, 47.063824), .Names = c("xmin", "ymin", "xmax", "ymax"
), class = "bbox"), crs = structure(list(epsg = NA_integer_,
proj4string = NA_character_), .Names = c("epsg", "proj4string"
), class = "crs"), n_empty = 0L)), .Names = c("LINEID", "ID_SEQ",
"geometry"), row.names = c(NA, -25L), class = c("sf", "data.frame"
), sf_column = "geometry", agr = structure(c(NA_integer_, NA_integer_
), .Names = c("LINEID", "ID_SEQ"), .Label = c("constant", "aggregate",
"identity"), class = "factor"))}
par(mfrow=c(1,3))
## SP - way
tstssp <- as(sfpoints, "Spatial")
tstssp <- SpatialLines(lapply(split(tstssp, tstssp$LINEID), function(x)
Lines(list(Line(coordinates(x))), x$LINEID[1L])))
plot(tstssp, col=1:2, lwd=3, main="SP-Desired Result")
## SF - way ???
tst <- sfpoints %>%
group_by(LINEID) %>%
st_coordinates() %>%
st_linestring()
plot(st_geometry(tst), col=1:2 , main="SF-Wrong Result")
tst <- sfpoints %>%
group_by(LINEID) %>%
summarise() %>%
st_cast("LINESTRING")
plot(st_geometry(tst), col=1:2, main="SF-Wrong Result")
【问题讨论】：
-
几天前我遇到了一个非常相似的问题。在
sfgithub 上有一些问题报告：github.com/r-spatial/sf/issues/325 和github.com/r-spatial/sf/issues/321。st_union似乎没有注意数据框中的顺序，而是根据彼此最近的点进行排序。 -
在我的情况下，我最终按 ID 拆分数据帧，并使用了
gtfsr包中的shapes_df_as_sfg，然后将它们重新绑定在一起并转换为线串。不幸的是，我无法适应你的科幻 -
感谢您的回复。我已经看到了那些 github 问题，但它们对我的情况没有帮助。我唯一能做的就是更改为 sp 结构，将点拆分为线，然后转换回 sf.. 但我没有看到
st_union采用最近的下一个点，因为这会在情节 3 中产生正确的结果，所以我仍然想知道它如何将这些点结合在一起，从而输出这种锯齿形结构。 -
我错误地说它是最近的点，我认为它实际上是通过增加经度值来排序的，这就是你最终得到之字形的方式
-
这可以解释奇怪的形状。太糟糕了，我不能通过 ID_SEQ 告诉他正确的顺序..