[R-sig-Geo] Problem finding the centroid of a polygon with get.Pcent

[Mauricio Zambrano]

Dear Roger,

2008/5/23 Roger Bivand <Roger.Bivand at nhh.no>:
> On Fri, 23 May 2008, Mauricio Zambrano wrote:
>
>> Dear List,
>>
>> For finding the centroid of a catchment, I tried with the function:
>>
>> get.Pcent(gallego_catchment)
>>
>> but I got the following  error:
>>
>> Error en get.Pcent(gallego_catchment) : not a Map object
>>
>> However, I checked the class:
>>
>>> class(gallego_catchment)
>>
>> [1] "SpatialPolygonsDataFrame"
>> attr(,"package")
>> [1] "sp"
>
> Just say coordinates(gallego_catchment) to extract the "centroid" -
> labelpoint - for each Polygons object - the coordinates() method retrieves
> the label points.

Does it mean that the coordinates of the centroid are stored within
the shapefile, or  are they computed by the coordinates command ?.

Note that a "Map" object is not a
> "SpatialPolygonsDataFrame" object, and indeed "Map" objects are only used
> internally in some limited settings. You are muddling up classes. Installing
> rgdal properly would make sure you only saw the "SpatialPolygonsDataFrame"
> object, which is all you need.
>
Thanks for your answer and for clarifying my confusion about the Map
and SpatialPolygonsDataFrame class.
I have gdal 1.5.1-3 installed in my linux system, and when I tried to
load the "rgdal" library whitin R, I got the following error:

library(rgdal)
Error in fun(...) :
	GDAL Error 1: libgrass_I.so: no se puede abrír el archivo de objeto
compartido: No existe el fichero ó directorio

Error : .onLoad failed in 'loadNamespace' for 'rgdal'
Error: package/namespace load failed for 'rgdal'

I uninstalled the rgdal package, then installed again with:

install.packages("rgdal", dependencies = TRUE)	

but when I tried to load it again, I got the same error:

library(rgdal)
Loading required package: sp
Error in fun(...) :
	GDAL Error 1: libgrass_I.so: no se puede abrír el archivo de objeto
compartido: No existe el fichero ó directorio

Error : .onLoad failed in 'loadNamespace' for 'rgdal'
Error: package/namespace load failed for 'rgdal'

Do you have any idea about what could be my problem ?.

Thanks in advance

Mauricio

> Roger
>
>>
>> and I read the catchment boundary with:
>>
>> p4s <- CRS("+proj=utm +zone=30 +ellps=intl +units=m +no_defs")
>> # reading the boundary of the gallego catchment
>> library(maptools)
>> gallego_catchment <- readShapePoly("only_gallego_catchment.shp",
>> proj4string=p4s)
>>
>> Could somebody tell me if I'm doing something wrong?
>>
>> Thanks in advance
>>
>> Mauricio
>>
>>
>
> --
> 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
>
>