[R-sig-Geo] Replace a polygon by a new one of a SpatialPolygonsDataFrame

[Barry Rowlingson]

This works: create a new SpatialPolygonsDataFrame with just your new
geometry, copying the data from the old geometry, and then use cbind
and selection to create a new object:

theNew <- list(Polygons(list(Polygon(cbind(c(r[1,1],r[1,1],r[1,2],r[1,2],r[1,1]),c(r[2,1],r[2,2],r[2,2],r[2,1],r[2,1])))),
theID))
# just the ws row:
ws = wrld_simpl[237,]

# create a single row SPDF with new geom and old data:
wsb = SpatialPolygonsDataFrame(SpatialPolygons(theNew),data=ws at data)
proj4string(wsb)=proj4string(wrld_simpl)

# create complete new object:
wrld_simpl=rbind(wrld_simpl[-237,], wsb)

(note I had to change your construction of theNew since I got a
"polygon not closed" error).

as written the wsb row will be at the end, but you can tweak that with
some clever subsetting and indexing.


Barry



On Tue, May 12, 2015 at 6:07 PM, Hans-Jörg Bibiko <bibiko at eva.mpg.de> wrote:
> Dear all,
>
> I believe I do not see the forest for the trees. Since hours I've been trying something very basal. Maybe someone could give me an hint.
>
> Imaging the following scenario:
>
> I have a SpatialPolygonsDataFrame which contains lots of spatial polygons and associated data. Now I'd like to replace one polygon (a list of a Polygons-class) of that SpatialPolygonsDataFrame by a new one. How can I do this.
>
>
> Here is a very naïve, reproducible example:
>
> Task: Replace the Polygons showing "Western Sahara" of the data set "world_simpl" by a new one which only shows its rectangle (bbox).
>
> ---------------------------------------------------------------
> library(maptools)
> data(wrld_simpl)
>
> #wrld_simpl[237,] := Western Sahara
> r <- bbox(wrld_simpl[237,])
>
> theID <- wrld_simpl[237,]@polygons[[1]]@ID
>
> # create the new spatial polygon (a rectangle of bbox coordinates)
> theNew <- list(Polygons(list(Polygon(cbind(c(r[1,1],r[1,1],r[1,2],r[1,2]),c(r[2,1],r[2,2],r[2,2],r[2,1])))), theID))
>
> # now I thought that one can do the this:
> wrld_simpl[237,]@polygons <- theNew
>
> ---------------------------------------------------------------
>
> But the last line does't work albeit the class and list resp. are of the same structure. :/
>
> Old:
>> str(wrld_simpl[237,]@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] -13.1 24.7
>   .. .. .. .. ..@ area   : num 24
>   .. .. .. .. ..@ hole   : logi FALSE
>   .. .. .. .. ..@ ringDir: int 1
>   .. .. .. .. ..@ coords : num [1:23, 1:2] -15.7 -17 -17.1 -16.9 -16.7 ...
>   .. ..@ plotOrder: int 1
>   .. ..@ labpt    : num [1:2] -13.1 24.7
>   .. ..@ ID       : chr "ESH"
>   .. ..@ area     : num 24
>
>
> New:
>> str(theNew)
> 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] -14.3 10.5
>   .. .. .. .. ..@ area   : num 159
>   .. .. .. .. ..@ hole   : logi FALSE
>   .. .. .. .. ..@ ringDir: int 1
>   .. .. .. .. ..@ coords : num [1:5, 1:2] -17.05 -17.05 -8.67 -8.67 -17.05 ...
>   .. ..@ plotOrder: int 1
>   .. ..@ labpt    : num [1:2] -14.3 10.5
>   .. ..@ ID       : chr "ESH"
>   .. ..@ area     : num 159
>
> ?
>
> I really appreciate any help someone can provide!
>
> Cheers, Hans
>
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