[R] Extract a slot value from a 'SpatialPolygons' class object

[Xavier Hoenner]

Dear R-users,

I’m currently trying to extract the value of a slot (area) but can’t find out 
how to do that.

>str(overlperc)

List of 1
 $ :Formal class 'SpatialPolygons' [package "sp"] with 4 slots
  .. ..@ polygons   :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] 709374 -1507888
  .. .. .. .. .. .. .. ..@ area   : num 11542604
  .. .. .. .. .. .. .. ..@ hole   : logi FALSE
  .. .. .. .. .. .. .. ..@ ringDir: int 1
  .. .. .. .. .. .. .. ..@ coords : num [1:23, 1:2] 706840 706657 706840 707294 
707585 ...
  .. .. .. .. .. .. .. .. ..- attr(*, "dimnames")=List of 2
  .. .. .. .. .. .. .. .. .. ..$ : chr [1:23] "1" "2" "3" "4" ...
  .. .. .. .. .. .. .. .. .. ..$ : chr [1:2] "x" "y"
  .. .. .. .. ..@ plotOrder: int 1
  .. .. .. .. ..@ labpt    : num [1:2] 709374 -1507888
  .. .. .. .. ..@ ID       : chr "1"
  .. .. .. .. ..@ area     : num 11542604
  .. ..@ plotOrder  : int 1
  .. ..@ bbox       : num [1:2, 1:2] 706657 -1509411 711710 -1506189
  .. .. ..- attr(*, "dimnames")=List of 2
  .. .. .. ..$ : chr [1:2] "x" "y"
  .. .. .. ..$ : chr [1:2] "min" "max"
  .. ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
  .. .. .. ..@ projargs: chr NA

I’d like to extract the area value so as to be able to use this value for 
further analysis. Here’s below what I get when I only type the name of the 
object:

>overlperc

[[1]]
An object of class "SpatialPolygons"
Slot "polygons":
[[1]]
An object of class "Polygons"
Slot "Polygons":
[[1]]
An object of class "Polygon"
Slot "labpt":
[1]   709374 -1507888

Slot "area":
[1] 11542604

Slot "hole":
[1] FALSE

Slot "ringDir":
[1] 1

Slot "coords":
          x        y
1  706839.8 -1508654
2  706657.2 -1508029
3  706839.8 -1507634
4  707293.6 -1507284
5  707584.7 -1507174
6  708329.6 -1506851
7  709013.3 -1506539
8  709074.5 -1506513
9  709819.5 -1506189
10 710564.4 -1506289
11 711021.1 -1506539
12 711309.3 -1506769
13 711642.0 -1507284
14 711709.8 -1508029
15 711309.3 -1508690
16 711246.4 -1508774
17 710564.4 -1509347
18 709819.5 -1509411
19 709074.5 -1509277
20 708329.6 -1509072
21 707584.7 -1509039
22 706927.4 -1508774
23 706839.8 -1508654



Slot "plotOrder":
[1] 1

Slot "labpt":
[1]   709374 -1507888

Slot "ID":
[1] "1"

Slot "area":
[1] 11542604



Slot "plotOrder":
[1] 1

Slot "bbox":
         min        max
x   706657.2   711709.8
y -1509411.3 -1506189.0

Slot "proj4string":
CRS arguments: NA

I’m stuck and spent more or less the whole afternoon online trying to find a 
solution to my problem but I couldn’t find anything. I look forward to hear 
from some of you. Thanks in advance for your kind help.

Have a good weekend.

Cheers


Xavier


Xavier Hoenner
PhD Student
Nesting Ecology, Migrations and Diving Behaviour of Hawksbill Turtles
School of Environmental Research, Charles Darwin University
Darwin, NT 0909
Ph: (08)8946.7721
email:xavier.hoen...@cdu.edu.au

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