[R] Retrieving Vertices Coordinates from SpatialPolygons

[Barry Rowlingson]

On Sat, Mar 21, 2009 at 12:33 PM, Enrico R. Crema <e.crema at ucl.ac.uk> wrote:
> Dear List,
>
> I'm trying to use different R packages for my Teaching Assistantship
> classes. And I cam out to an (apparently) very simple problem. I would
> like to retrieve the vertices coordinate of a SpatialPolygon data. I
> know this is stored in the "coords" slot, but I can't get access to
> it! I tried to coerce the SpatialPolygon into a data.frame but it
> doesn't work. Want I want is just a list of x and y coordinates of my
> polygon vertices without doing the workflow in GRASS!!!

There's not really such thing as a SpatialPolygon - there's only
SpatialPolygons - so maybe you've got a SpatialPolygons object with
only one feature in it..

 str(thing) will help here. This is what I get if I str() something
that is a SpatialPolygons object with only one feature in it:

> str(poly)
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] 1 10
  .. .. .. .. .. .. ..@ area   : num 1
  .. .. .. .. .. .. ..@ hole   : logi FALSE
  .. .. .. .. .. .. ..@ ringDir: int 1
  .. .. .. .. .. .. ..@ coords : num [1:5, 1:2] 0.5 0.5 1.5 1.5 0.5
9.5 10.5 10.5 9.5 9.5
  .. .. .. .. .. .. .. ..- attr(*, "dimnames")=List of 2
  .. .. .. .. .. .. .. .. ..$ : chr [1:5] "s1" "s1" "s1" "s1" ...
  .. .. .. .. .. .. .. .. ..$ : chr [1:2] "x" "y"
  .. .. .. ..@ plotOrder: int 1
  .. .. .. ..@ labpt    : num [1:2] 1 10
  .. .. .. ..@ ID       : chr "g1"
  .. .. .. ..@ area     : num 1
  ..@ plotOrder  : int 1
  ..@ bbox       : num [1:2, 1:2] 0.5 9.5 1.5 10.5
  .. ..- attr(*, "dimnames")=List of 2
  .. .. ..$ : chr [1:2] "r1" "r2"
  .. .. ..$ : chr [1:2] "min" "max"
  ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
  .. .. ..@ projargs: chr NA

 I'll explain, stepping through the structure breadth first, and backwards:

 @proj4string is the coordinate reference system slot
 @bbox is the bounding box slot
 @plotOrder is the order to plot the polygons
 @polygons is the list of Polygons objects.

 @polygons[[1]] is the first (and in this case, only) feature. It is
an object of class 'Polygons' (non-spatial, since there's no
@proj4string in this part of the structure).

 @polygons[[1]]@Polygons is the 'Polygons' slot of class 'Polygons',
and is a list of rings that make up the feature.

 @polygons[[1]]@Polygons[[1]] is an object of class 'Polygon'.

 @polygons[[1]]@Polygons[[1]]@coords is the coordinates of the Polygon:

> poly at polygons[[1]]@Polygons[[1]]@coords
     x    y
s1 0.5  9.5
s1 0.5 10.5
s1 1.5 10.5
s1 1.5  9.5
s1 0.5  9.5

 It's complex because the SpatialPolygons class can hold multiple
features (such as administrative subdivisions), and each feature can
be made from multiple rings (islands, holes). It seems overkill when
all you really want is to store a single simple closed polygon though!
But now you know how to get it. There may be methods for some of these
@slot calls.

 Personally, I forget this everytime, but str() is immensely useful.
As is a copy of the Bivand/Pebesma/Gomez-Rubio book!

Barry