x@polygons[2] 数据结构是：
str(x@polygons[2])
List of 1
$ :Formal class 'Polygons' [package "sp"] with 5 slots
.. ..@ Polygons :List of 1
.. .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
.. .. .. .. ..@ labpt : num [1:2] 1264936 6311551
.. .. .. .. ..@ area : num 1.32e+10
.. .. .. .. ..@ hole : logi FALSE
.. .. .. .. ..@ ringDir: int 1
.. .. .. .. ..@ coords : num [1:142, 1:2] 1226011 1228542 1233476 1237278 1241942 ...
.. ..@ plotOrder: int 1
.. ..@ labpt : num [1:2] 1264936 6311551
.. ..@ ID : chr "1"
.. ..@ area : num 1.32e+10

x@polygons[2]
[[1]]
An object of class "Polygons"
Slot "Polygons":
[[1]]
An object of class "Polygon"
Slot "labpt":
[1] 1264936 6311551

Slot "area":
[1] 13202433208

Slot "hole":
[1] FALSE

Slot "ringDir":
[1] 1

Slot "coords":
[,1] [,2]
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[2,] 1228542 6374219
[3,] 1233476 6369585
[4,] 1237278 6365661
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[142,] 1226011 6375310

Slot "plotOrder":
[1] 1

Slot "labpt":
[1] 1264936 6311551

Slot "ID":
[1] "1"

Slot "area":
[1] 13202433208