On Thu, 12 Feb 2009, Don MacQueen wrote:
I feel a bit lazy asking for help, as I know I can do this -- but since I
don't work with spatial polygons frequently, I find assembling objects
consisting of nested polygons to be a bit challenging.
It can be done, so this approach from the other side may not cater for the
specifics. Using the NC counties, construct outbox:
library(maptools)
xx <- readShapeSpatial(system.file("shapes/sids.shp", package="maptools")[1],
IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
library(splancs)
crds <- sbox(bboxx(bbox(xx)))
crds <- rbind(crds, crds[1,])
outbox <- Polygon(crds, hole=FALSE)
# sample a (much) larger number in outbox
zz <- sample.Polygon(outbox, n=2000, type="random")
plot(SpatialPolygons(list(Polygons(list(outbox), ID="box"))))
points(zz, pch=3, cex=0.5)
plot(xx, add=TRUE, border="red")
# overlay the points and the interior polygons and drop those inside
o <- overlay(as(xx, "SpatialPolygons"), zz)
zzz <- zz[is.na(o)]
zzzz <- zzz[sample(nrow(coordinates(zzz)), 500)]
points(zzzz, pch=3, cex=0.5, col="green")
which does something like your description. It ought to work if there are
holes in your input polygons too.
It is possible to flatten the input Polygons objects into a single
Polygons object, add the enclosing Polygon object to the list, and pass to
checkPolygonsHoles(), but maybe the above is simpler.
Hope this helps,
Roger
I have a Spatial Polygons Data Frame object named "s2poly" (read in from a
polygons shapefile). The first portion of the output from str(s2poly) is
shown here:
Formal class 'SpatialPolygonsDataFrame' [package "sp"] with 5 slots
..@ data :'data.frame': 1173 obs. of 11 variables:
.. ..$ OBJECTID : num [1:1173] 3 12 13 77 78 79 80 81 82 83 ...
.. ..$ ENTITY : Factor w/ 0 levels: NA NA NA NA NA NA NA NA NA NA ...
.. ..$ HANDLE : Factor w/ 1173 levels "10002","10008",..: 348 349 350
351 352 353 354 355 356 357 ...
.. ..$ LAYER : Factor w/ 15 levels "ADMIN_MAP","B-BLDG-LINE",..: 14 8 8
8 8 11 11 11 11 9 ...
.. ..$ COLOR : num [1:1173] 252 4 4 4 4 200 200 200 200 7 ...
.. ..$ LINETYPE : Factor w/ 3 levels "Continuous","DASHED",..: 1 1 1 1 1 1
1 1 1 1 ...
.. ..$ ELEVATION : num [1:1173] 0 0 0 0 0 0 0 0 0 0 ...
.. ..$ THICKNESS : num [1:1173] 0 0 0 0 0 0 0 0 0 0 ...
.. ..$ TEXT : Factor w/ 0 levels: NA NA NA NA NA NA NA NA NA NA ...
.. ..$ SHAPE_AREA: num [1:1173] 10.8 32549.7 16571.2 37.7 59.2 ...
.. ..$ SHAPE_LEN : num [1:1173] 19 932.6 608 77.4 120.3 ...
.. ..- attr(*, "data_types")= chr [1:11] "N" "C" "C" "C" ...
..@ polygons :List of 1173
.. ..$ :Formal class 'Polygons' [package "sp"] with 5 slots
.. .. .. ..@ Polygons :List of 1
.. .. .. .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
.. .. .. .. .. .. ..@ labpt : num [1:2] 1653186 431813
.. .. .. .. .. .. ..@ area : num 10.8
.. .. .. .. .. .. ..@ hole : logi FALSE
.. .. .. .. .. .. ..@ ringDir: int 1
.. .. .. .. .. .. ..@ coords : num [1:8, 1:2] 1653190 1653190 1653187
1653188 1653183 ...
.. .. .. ..@ plotOrder: int 1
.. .. .. ..@ labpt : num [1:2] 1653186 431813
.. .. .. ..@ ID : chr "0"
.. .. .. ..@ area : num 10.8
I suspect, but have not confirmed, that some of the instances of Polygons
objects have length > 1.
I have also created a simple polygon named "s2bound" that surrounds all of
the polygons in s2poly.
I need a single object that I can give to spsample() in which all of the
polygons in s2poly are holes within s2bound.
I don't need to keep any of the attributed data in @data, so I believe I can
simplify to a SpatialPolygons object.
My basic request for assistance is this:
- can it be done with a modest number of high levels functions? (and if
so, I think I need to be shown how)
- or will I need to write nested loops of @Polygons within @polygons to
change @hole to TRUE in every polygon, in order construct my final polygon?
Thanks
-Don
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: roger.biv...@nhh.no
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