[R-sig-Geo] transformin multipart shapefile to singlepart shapefile

[Jean-Luc DUPOUEY]

I faced the same question recently: how to tranform a multipart polygon layer
into single polygons? Here is a little bit more general function to do that.
It keeps the initial ID of the polygons, adding a sequence number. The
initial plot order and projection information are also kept. 

But pay attention, holes are automatically converted into islands, because
holes cannot be single polygons.

I am interested in improving this function. Thanks in advance,

JL Dupouey

#JL Dupouey INRA-Nancy 2015/01/31

#convert a multipart SpatialPolygons object to a single part SpatialPolygons
object
#plot order: keeps the initial plot order 
#p4s: keeps the initial p4s, if not NA
#ID: keeps the initial ID of the multipart polygons, and add a numeric
increasing index from 1 to n 
# in order of presence of the single polygons wich compose the multipart
polygon, if there is more then one part

multitoone=function(mpart)

{
#vector of number of elementary polygons in each multipart polygon (for
calculation of plot order, and initialization of islands)
nbpoly=sapply(mpart at polygons,function(x){length(x at Polygons)})

#initialize the list of single Polygons to be built
islands=vector("list",sum(nbpoly))
#initialize the vector plot order of single polygons to be built
plotorder=vector("integer",sum(nbpoly))

#index of current single Polygons
k=0

#loop on all Polygons of the object

for (i in 1:length(mpart))
{

#current multiple polygon
pols=mpart at polygons[[i]]@Polygons
ID=mpart at polygons[[i]]@ID

#number of polygons to plot before the current multiple one
prev=sum(nbpoly[mpart at plotOrder<mpart at plotOrder[i]])

#loop on each elementary polygon of the current multiple polygon

for (j in 1:length(pols)) 
 { k=k+1
   IDs=ifelse(length(pols)>1,paste(ID,'-',j,sep=''),ID) 
   islands[[k]] = Polygons(list(pols[[j]]), IDs)
  
plotorder[k]=rank(mpart at polygons[[i]]@plotOrder,ties.method="first")[j]+prev
}
}#end on loop on all Polygons of the object

multitoone = SpatialPolygons(islands,pO=plotorder)
if (!is.na(proj4string(mpart))) proj4string(multitoone)=proj4string(mpart) 

return(multitoone)
}

 



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