[R-sig-Geo] points to lines and/or polygons

[Roger Bivand]

On Wed, 27 Jan 2010, Agustin Lobo wrote:

> Thanks, one clarification:
> If I do:
>> data(kola.background)
>> xy <- kola.background$coast
>> names(xy) <- c("x", "y")
>> class(xy)
> [1] "data.frame"
>
> How is it that I can run successfully:
> coast <- map2SpatialLines(xy)
>
> if the help page states that the argument must be of "map" class?:
> map2SpatialLines(map, IDs=NULL, proj4string = CRS(as.character(NA)))
> Arguments
> mapa map object defined in the maps package and returned by the map function
> ?

The maps package is the main (geographical) source using the legacy S 
format, where lines (or polygons) are separated by NA rows in a two-column 
matrix. This is described in ?lines. Think of the simplest format for a 
pen plotter, where NA picks up the pen. As the format is used by other 
sources (including base graphics), the function doesn't check that the 
class is "map", but converting from this class is its main use.

Roger

>
> Also,
>> str(coast,max.level=2)
> Formal class 'SpatialLines' [package "sp"] with 3 slots
>  ..@ lines      :List of 142
>  ..@ bbox       : num [1:2, 1:2] 378389 7388684 865816 7900281
>  .. ..- attr(*, "dimnames")=List of 2
>  ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
>
> which is the criteria by which  map2SpatialLines() starts a new line?
>
> Just in case anybody might be curious, after doing what you suggested, I do
> the following to save as shape:
> require(rgdal)
> str(coast,max.level=2) #from which I know there are 142 Lines
> coastdata <- data.frame(ID=1:142,name=rep("",142),stringsAsFactors=F)
> row.names(coastdata) <- 1:142
> coast <- SpatialLinesDataFrame(sl=coast, data=coastdata, match.ID = TRUE)
> writeOGR(coast,dsn="Kolacoast",layer="Kolacoasts",driver="ESRI Shapefile")
>
> Agus
>
> 2010/1/26 Roger Bivand <Roger.Bivand at nhh.no>
>
>> On Tue, 26 Jan 2010, Agustin Lobo wrote:
>>
>>  Hi!
>>> Package StatDA
>>> provides background geoinformation as a set of coordinates:
>>>
>>>> str(kola.background)
>>>>
>>> List of 4
>>> $ boundary:'data.frame':       50 obs. of  2 variables:
>>>  ..$ V1: num [1:50] 388650 388160 386587 384035 383029 ...
>>>  ..$ V2: num [1:50] 7892400 7881248 7847303 7790797 7769214 ...
>>> $ coast   :'data.frame':       6259 obs. of  2 variables:
>>>  ..$ V1: num [1:6259] 438431 439102 439102 439643 439643 ...
>>>  ..$ V2: num [1:6259] 7895619 7896495 7896495 7895800 7895542 ...
>>> $ borders :'data.frame':       504 obs. of  2 variables:
>>>  ..$ V1: num [1:504] 417575 417704 418890 420308 422731 ...
>>>  ..$ V2: num [1:504] 7612984 7612984 7613293 7614530 7615972 ...
>>> $ lakes   :'data.frame':       6003 obs. of  2 variables:
>>>  ..$ V1: num [1:6003] 547972 546915 NA 547972 547172 ...
>>>  ..$ V2: num [1:6003] 7815109 7815599 NA 7815109 7813873 ...
>>>
>>> is there any spatial function aready availale to convert these coordinates
>>> into Spatial Lines and Spatial Polygons?
>>>
>>
>> This seems to work at least for SpatialLines - for polygons, the rings may
>> need to be closed:
>>
>> library(StatDA)
>> data(kola.background)
>> xy <- kola.background$boundary
>> names(xy) <- c("x", "y")
>> library(maptools)
>> bdy <- map2SpatialLines(xy)
>> plot(bdy)
>>
>> The data are in the legacy S format (like the maps package).
>>
>> Roger
>>
>>
>>
>>> Thanks
>>>
>>> Agus
>>>
>>>        [[alternative HTML version deleted]]
>>>
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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
>>
>>
>

-- 
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