[R-sig-Geo] unionSpatialPolygons - single polygon instead of multipart polygon

[hadley wickham]

Hi All,

I'm trying to figure out exactly what output unionSpatialPolygons
gives me - it looks like it merges my singleton polygons into
multi-polygons (see output of str below), but what I really want is
just a single singleton polygon, containing the boundary of the
unioned areas.  How can I get that?

> str(unioned[4]@polygons[[1]])
Formal class 'Polygons' [package "sp"] with 5 slots
  ..@ Polygons :List of 3
  .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
  .. .. .. ..@ labpt  : num [1:2] -99.6  28.6
  .. .. .. ..@ area   : num 0.063
  .. .. .. ..@ hole   : logi FALSE
  .. .. .. ..@ ringDir: int 1
  .. .. .. ..@ coords : num [1:151, 1:2] -99.8 -99.7 -99.7 -99.6 -99.6 ...
  .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
  .. .. .. ..@ labpt  : num [1:2] -98.2  28.5
  .. .. .. ..@ area   : num 0.112
  .. .. .. ..@ hole   : logi FALSE
  .. .. .. ..@ ringDir: int 1
  .. .. .. ..@ coords : num [1:131, 1:2] -98 -98 -98 -98 -98 ...
  .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
  .. .. .. ..@ labpt  : num [1:2] -96.7  28.5
  .. .. .. ..@ area   : num 0.0558
  .. .. .. ..@ hole   : logi FALSE
  .. .. .. ..@ ringDir: int 1
  .. .. .. ..@ coords : num [1:190, 1:2] -96.8 -96.8 -96.8 -96.8 -96.8 ...
  ..@ plotOrder: int [1:3] 2 1 3
  ..@ labpt    : num [1:2] -98.2  28.5
  ..@ ID       : chr "007"
  ..@ area     : num 0.231


I've included my full code below to give more of a flavour of what I'm
trying to do - simplify the boundaries of TX counties and then get the
data into a regular R data frame.  This doesn't work at the moment,
because I'm only getting the first part of the multipart polygons.

counties <- readShapeSpatial("tx-counties")

attr <- as.data.frame(counties)
names(attr) <- tolower(names(attr))
attr <- attr[c("fips", "name", "area", "perimeter")]

polys <- split(row.names(attr), attr$fips)

cp <- polygons(counties)
unioned <- unionSpatialPolygons(cp, invert(polys))

coords <- function(x) x at polygons[[1]]@Polygons[[1]]@coords
ccoords <- lapply(seq_len(254), function(i) coords(unioned[i]))

id <- function(x) x at polygons[[1]]@ID
names(ccoords) <-  sapply(seq_len(254), function(i) id(unioned[i]))

cdf <- do.call("rbind", lapply(seq_along(ccoords), function(i) {
  df <- as.data.frame(ccoords[[i]])
  names(df) <- c("x", "y")
  df <- as.data.frame(dp(df, 0.01))
  df$id <- names(ccoords)[i]
  df
}))


Hadley

-- 
http://had.co.nz/