[R-sig-Geo] Multiple polygons

[Roger Bivand]

On Wed, 14 Oct 2009, Rui Catarino wrote:

> Hello again,
>
> Thank you for such a quick reply.
>
> You were right, it seems a bit obscure so following your suggestion I
> attach a small example of what looks like my table. The list is in excel
> format and I usually use the package xlsReadWrite to read the excel
> files.

After converting to CSV (no Excel here):

Example.df <- read.csv("Example.csv")
ldf <- split(Example.df, Example.df$Polygon_n)
# split into list on polygon ID
lpls <- lapply(ldf, function(x) {
   crds <- cbind(x$Long, x$Lat)
   crds <- crds[x$order,]
   crds <- rbind(crds, crds[1,])
   ID <- as.character(unique(x$Polygon_n))
   Polygons(list(Polygon(crds)), ID=ID)
})
# lapply to make a list of Polygons objects
lvdf <- lapply(ldf, function(x) {
   rn <- as.character(unique(x$Polygon_n))
   StartDate <- strptime(unique(as.character(x$StartDate)), format="%m/%d/%Y")
   CloseDate <- strptime(unique(as.character(x$CloseDate)), format="%m/%d/%Y")
   AnalysisDate <- strptime(unique(as.character(x$AnalysisDate)),
     format="%m/%d/%Y")
   data.frame(StartDate=StartDate, CloseDate=CloseDate,
     AnalysisDate=AnalysisDate, row.names=rn)
})
vdf <- do.call("rbind", lvdf)
# lapply to extract the unique values and make them into POSIXlt objects 
# as they are dates, then do.call("rbind", ...) to stick the n single
# line data.frames together
spdf <- SpatialPolygonsDataFrame(SpatialPolygons(lpls,
   proj4string=CRS("+proj=longlat +datum=WGS84")), data=vdf)
# assemble the list of Polygons objects and matching data.frame
# as a SpatialPolygonsDataFrame

All the *apply() functions are really user friendly and save very much 
more time than that needed to learn them. Think of lapply() and sapply() 
like a for loop along a vector, usually along a list, doing something to 
each component - here custom functions. Starting on a small, 
representative example usually helps to get the incantation right.

Note that the orders were all redundant here, so adjustment may be needed 
there.

Hope this helps,

Roger


>
> Once again thank you for your trouble
> Rui
>
> -----Original Message-----
> From: Roger Bivand [mailto:Roger.Bivand at nhh.no]
> Sent: 14 October 2009 14:30
> To: Rui Catarino
> Cc: r-sig-geo at stat.math.ethz.ch
> Subject: Re: [R-sig-Geo] Multiple polygons
>
> 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
>>
>>
>>
>>
>> 	[[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