如何在执行st_union()操作后将多面几何体分成几个多边形对象？

Question

我有一组多边形,其中一些多边形相交和/或触摸(公共边框).我正在使用R的sf包对多边形执行操作.到目前为止,我的方法是使用sf::st_union()连接相邻和交叉多边形的方法,但它也将所有多边形组合成 MULTIPOLYGON几何.我想将每个多边形分隔为一个sf(data.frame)类,其中每个多边形对象在一个行中显示为一行data.frame

我在下面举例说明.我首先创建一个示例数据集:

    # Creating four example polygons, of which two (two squares) are neighbors:

    p1 <- rbind(c(0,0), c(1,0), c(3,2), c(2,4), c(1,4), c(0,0))
    pol1 <-st_polygon(list(p1))
    p2 <- rbind(c(3,0), c(4,0), c(4,1), c(3,1), c(3,0))
    pol2 <-st_polygon(list(p2))
    p3 <- rbind(c(4,0), c(4,1), c(5,1), c(5,0),c(4,0))
    pol3 <-st_polygon(list(p3))
    p4 <- rbind(c(3,3), c(4,2), c(4,3), c(3,3))
    pol4 <-st_polygon(list(p4))

    d = data.frame(some_attribute = 1:4)
    d$geometry = st_sfc(pol1,pol2,pol3,pol4)
    df = st_as_sf(d)

    class(df)
    #[1] "sf"         "data.frame"

    df
    # Simple feature collection with 4 features and 1 field
    # geometry type:  POLYGON
    # dimension:      XY
    # bbox:           xmin: 0 ymin: 0 xmax: 5 ymax: 4
    # epsg (SRID):    NA
    # proj4string:    NA
    # some_attribute                       geometry
    # 1              1 POLYGON((0 0, 1 0, 3 2, 2 4...
    # 2              2 POLYGON((3 0, 4 0, 4 1, 3 1...
    # 3              3 POLYGON((4 0, 4 1, 5 1, 5 0...
    # 4              4 POLYGON((3 3, 4 2, 4 3, 3 3))

plot(df) 得到:

然后我执行一个st_union()操作,将所有相交或触摸的多边形几何(上面的两个方块)组合成一个:

    df_union <- df %>% st_union() 
    df_union
    # Geometry set for 1 feature 
    # geometry type:  MULTIPOLYGON
    # dimension:      XY
    # bbox:           xmin: 0 ymin: 0 xmax: 5 ymax: 4
    # epsg (SRID):    NA
    # proj4string:    NA
    # MULTIPOLYGON(((3 3, 4 3, 4 2, 3 3)), ((4 0, 3 0...

plot(df_union) 结果是:

如上所示,结果df_union是MULTIPOLYGON仅具有一行的几何.我想执行一个操作,将每个多边形分成几何形状,如上图所示,但产生了几个多边形对象,相当于:

    # Simple feature collection with 4 features and 1 field
    # geometry type:  MULTIPOLYGON
    # dimension:      XY
    # bbox:           xmin: 0 ymin: 0 xmax: 5 ymax: 4
    # epsg (SRID):    NA
    # proj4string:    NA
    # some_attribute                       geometry
    # 1              1 POLYGON((0 0, 1 0, 3 2, 2 4...
    # 2              2 POLYGON((3 0, 4 0, 5 1, 5 0...
    # 3              3  POLYGON((3 3, 4 2, 4 3, 3 3))

我怎么能用这个sf包呢？

Answer 1

感谢Edzer提供的答案,并为我们提供了很棒的sf套餐!

正如Edzer执行的那样,st_cast将MULTIPOLYGON对象转换为多个POLYGON对象:

    df_union_cast <- st_cast(df_union, "POLYGON")
    df_union_cast
    # Geometry set for 3 features 
    # geometry type:  POLYGON
    # dimension:      XY
    # bbox:           xmin: 0 ymin: 0 xmax: 5 ymax: 4
    # epsg (SRID):    NA
    # proj4string:    NA
    # POLYGON((3 3, 4 3, 4 2, 3 3))
    # POLYGON((4 0, 3 0, 3 1, 4 1, 5 1, 5 0, 4 0))
    # POLYGON((0 0, 1 4, 2 4, 3 2, 1 0, 0 0))