[R-sig-Geo] Multiple polygons

[Roger Bivand]

On Wed, 14 Oct 2009, Rui Catarino wrote:

> Hello to all,
>
> This is my first attempt to use GIS tools in R so I'm sorry if my 
> question is to na?ve, but the truth is that I've spent several hours 
> looking in forum's and packages for a solution with no joy.
>
> What I'm trying to achieve is to create multiple polygons (around 150) 
> based in a table with (x,y) coordinates. This table also as two extra 
> fields one which will be the code (numeric value) for each polygon. The 
> second field is the numeric order in which the points in each polygon 
> should be joined.
>

What do the input data look like? Is there a list of +/-150 data.frame 
objects, or a single data.frame object (read with read.table()). If the 
latter, use split() first to make a list of data.frames, where the list 
components will take the name of the ID column values (assumed all 
identical for coordinates belonging to the same polygon).

Once you've got the list, say lst, you'll use lapply() to build a list of 
Polygons objects, each built of a list with a single Polygon object. This 
needs the coordinates in the right order and closed, so something like:

crds <- cbind(obj$x, obj$y)
crds <- crds[obj$order,]
crds <- rbind(crds, crds[1,])

should get them re-ordered, and then ring-closed. Insert the names as ID 
as appropriate. If this seems obscure, provide a very simple example data 
set on a website - it is actually quite straightforward if the data are 
cleanly organised.

Hope this helps,

Roger

>
>
> Thanks in advance
>
> Rui
>
>
>
>
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>
>

-- 
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