[R-sig-Geo] conversion to polygons

[pedro at dpi.inpe.br]

Hi,

I also have same "problem". Sometimes I want to build a SpatialPolygons 
from a set of points (a matrix) representing the border of a region, 
and it has only one polygon.

SP <- SpatialPolygons(list(Polygons(list(Polygon(matrixdata))),ID=1)))

But ID in this case is not "necessary", and also the two levels of 
list. At least for me, it is very common, and I can cite geoR and 
splancs packages, where we have this kind of data. I think in this case 
we could have a simpler code, and we could create it directly from the 
matrix:

SP = SpatialPolygons(matrixdata)

or from the Polygon:

SP = SpatialPolygons(polygon)

The creation of the lists and ID could be hidden in SpatialPolygons-methods.R:

SpatialPolygons <- function(Srl, pO, proj4string=CRS(as.character(NA))) {
     if( is.matrix(Srl) ) Srl <- list(Polygons(list(Polygon(Srl))),ID=1))
     ...
}

What do you think of it?

Pedro

Quoting Roger Bivand <Roger.Bivand at nhh.no>:

> On Wed, 14 Jun 2006, Luis Ridao Cruz wrote:
>
>> R-Sig-Geo help,
>>
>> I have an objct of class "SpatialPointsDataFrame"
>> which I woul like to convert to polygon.
>>
>> I have used the following:
>>
>> test1<-Polygon(test)
>> test2<-SpatialPolygons(list(test),1)
>
> test2<-SpatialPolygons(list(Polygons(list(test)),ID=1)))
>
> should do it - there are two layers, because some "Polygons" objects with
> a single ID can have multiple "Polygon" object members.
>
> Roger
>
>> plot(test2,col=2,pbg="white")
>>
>> but I get en error message like this:
>>
>> Error in as.list(X) : cannot get a slot ("Polygons") from an object of
>> type "double"
>>
>> I also try ploting:
>>
>> plot(test1)
>>
>> which gives :
>>
>> Error in plot.window(xlim, ylim, log, asp, ...) :
>>         need finite 'xlim' values
>> In addition: Warning messages:
>> 1: no non-missing arguments to min; returning Inf
>> 2: no non-missing arguments to max; returning -Inf
>> 3: no non-missing arguments to min; returning Inf
>> 4: no non-missing arguments to max; returning -Inf
>>
>>
>>
>>
>> Thank you in advance
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>
>
> --
> 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
>
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