[R-sig-Geo] Changing Shapefile's Coordinates

[Wang, Kevin (SYD)]

Thanks Barry, it worked!

By the way, is there a way to "rescale" the map (other than enlarge plotting window once the map is drawn)?  I'm asking this because there are a few local government areas that are too small (and hence I can't see the text (coordinates) plotted on it).

Cheers
Kevin

-----Original Message-----
From: b.rowlingson at googlemail.com [mailto:b.rowlingson at googlemail.com] On Behalf Of Barry Rowlingson
Sent: Wednesday, 20 July 2011 9:21 PM
To: Wang, Kevin (SYD)
Cc: r-sig-geo at r-project.org
Subject: Re: [R-sig-Geo] Changing Shapefile's Coordinates

On Wed, Jul 20, 2011 at 12:31 AM, Wang, Kevin (SYD) <kevinwang at kpmg.com.au> wrote:
> Hi,
>
>
>
> I'm just wondering if it's possible, in R, to modify a shapefile's 
> coordinates?
>
>
>
> I've shapefiles for each state/territory of Australia, and in them 
> there are local government areas coordinates.  Using readShapePoly(), 
> I am able to read them in and plot each state with its local 
> government areas.
>
>
>
> However, I also want to be able to "merge" local government areas into 
> roughly north-east-south-west - i.e. draw a state then divide it 
> roughly into four quadrants (or divide it in other ways by merging 
> various local government areas).   Is there any easy way that I can 
> manipulate the shapefile to do this?
>

 Another job for rgeos!

 Can do something with gUnionCascaded?:

library(maptools);library(rgeos)
xx <- readShapeSpatial(system.file("shapes/sids.shp",
package="maptools")[1],IDvar="FIPSNO", proj4string=CRS("+proj=longlat
+ellps=clrk66"))

# plot it, label it:

 plot(xx)
 text(coordinates(xx),as.character(1:nrow(xx)))

# now suppose we want to merge these regions:

 z1 = c(10,24,9,71)

# we do:
zx=gUnionCascaded(xx[z1,])


# which gives us a SpatialPolygons object with (in this case) one ring:

 plot(zx,add=TRUE,col="#80808080")


Repeat for each of your regions and splat everything together as a SpatialPolygonsDataFrame if thats what you need.

Look out for digitising errors which result in slivers and other junk where adjacent polygons dont match up.

Barry


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