Message-ID: <Pine.LNX.4.64.0809020818470.24937@reclus.nhh.no>
Date: 2008-09-02T06:27:39Z
From: Roger Bivand
Subject: Using R spatial tools to merge polygons
In-Reply-To: <246640.20940.qm@web52408.mail.re2.yahoo.com>
On Mon, 1 Sep 2008, Lyndon Ang wrote:
> Good day all,
>
> I've only recently been introduced to the useful spatial tools available in R (and indeed to GIS in general!).?? I have found reading some of the posts on this mailing list quite useful, so am hoping that someone can help, or else let me know it can't be done!
>
> I have a map in Shapefile format, which I can read into R.??The
> shapefile is divided into regions.??What I want to do is merge the
> regions on the shapefile into larger areas.??The link between each of
> the individual shapefile regions and the larger regions is on a text
> file of the following format:
>
> Area_ID?????? Larger_ID
> 1?????? ?????? ?????? ?????? ?????? 1
> 2?????? ?????? ?????? ?????? ?????? 1
> 3?????? ?????? ?????? ?????? ?????? 1
> 4?????? ?????? ?????? ?????? ?????? 2
> 5?????? ?????? ?????? ?????? ?????? 2
>
> Is it possible for me to do this in R, without the use of other GIS software?
Yes, you can use unionSpatialPolygons() in maptools. The specific problem
is to add Area_ID and Larger_ID to the SpatialPolygonsDataFrame that you
read in. If Area_ID exists in shapefile, then something like:
library(maptools)
SPDF0 <- readShapeSpatial("my_shapefile.shp", IDvar="Area_ID")
my_ids <- read.table("my_textfile")
row.names(my_ids) <- as.character(my_ids$Area_ID)
SPDF1 <- spCbind(SPDF0, my_ids)
# if there is trouble here, it is because the IDs don't match
IDs <- as.character(SPDF1$Larger_ID)
SP_out <- unionSpatialPolygons(SPDF1, IDs)
giving you a SpatialPolygons object with no attribute data (look at
aggregate() for ways of merging rows in data.frame objects.
Roger
>
> Thanks in advance for any help that may be provided.
>
> Kind regards
> Lyndon
>
>
> Win a MacBook Air or iPod touch with Yahoo!7. http://au.docs.yahoo.com/homepageset
> [[alternative HTML version deleted]]
>
>
--
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.Bivand at nhh.no