[R-sig-Geo] Subsetting a spatial polygon dataframe with a for loop

[Edzer J. Pebesma]

I'm not a 100% sure, but it might be that you are using c(sub.cb.poly, 
i) which creates a list when the arguments are complicated structures. 
You may wish to use rbind on two (or more!) SpatialPolygonsDataFrame 
objects instead. Provided you have the latest sp from CRAN...

Hth,
--
Edzer

Andrew Niccolai wrote:
> Hi.  I had such great success last time, I thought I would ask for more
> assistance.  I have an object (cb.poly) of class "SpatialPolygonsDataFrame".
> I would like to subset this object using a for loop that essentially loops
> through polygon ID from a larger list.  The code looks like this:
>
> class(cb.poly)
> sub.cb.poly <- cb.poly[cb.poly$Plg_id == thresh.poly.id[[687]][1,],]
> polygons(sub.cb.poly)
> dissolveset <- thresh.poly.id[[687]]
> ilength <- nrow(dissolveset)
> for (i in 2:ilength){
> i <- cb.poly[cb.poly$Plg_id == thresh.poly.id[[687]][i,],]
> polygons(i)
> sub.cb.poly <- c(sub.cb.poly, i)
> }
> gg <- SpatialPolygons(sub.cb.poly,pO=1:length(sub.cb.poly))
>
> The object thresh.poly.id[[]] is a "list" class object that holds the unique
> polygon names that overlapped with my tree crown polygon (thanks Roger and
> Greg and others).
>
> The first time I call sub.cb.poly it is a "SpatialPolygonsDataFrame" class.
> I then try to convert to a polygon so that after I loop it through my list
> of appropriate polygon IDs I can reconvert to a "SpatialPolygonsDataFrame"
> class.  I am attempting this because after the for loop, my sub.cb.poly
> object is a "list" class.  I could not determine how to join elements from a
> loop into a "SpatialPolygonsDataFrame" class, so I thought I might convert
> to polygons and then use SpatialPolygons function (see gg object).  
>
> This is not working for me.  
>
> Ultimately, I want to create an object that is a subset of my original
> cb.poly object so that I can do a "unionSpatialPolygons" operation to create
> a merged or dissolved object. 
>
> Thanks for any assistance. 
>
>
> Andrew Niccolai
> Doctoral Candidate
> Yale School of Forestry
> 203-432-5144
>
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