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

[Mauricio Zambrano]

2008/5/27 Roger Bivand <Roger.Bivand at nhh.no>:
> On Tue, 27 May 2008, Mauricio Zambrano wrote:
>
>> 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 ?.
>
> The shapefile is a collection of files in your file-system. It is not
> present in the R workspace. It is read into a Spatial*DataFrame object,
> which in the case of SpatialPolygonsDataFrame objects, does return the
> precomputed label point of the largest constituent polygon for each
> observation when the coordinates() method is used.
>
>>
>> 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
>
> I already replied that you have a stale GRASS OGR plugin on your system,
> that GDAL can see, but where its dependency on libgrass_I.so is not
> fulfilled. I'm sure that no other GDAL programmes work. Make GDAL work first
> (even just gdalinfo --version) and rgdal will work. As I suggested before,
> you need to tread carefully when using binary versions of QGIS/GDAL etc.
>
> Roger

Thanks for your help. The problem was the "libgdal1-1.4.0-grass"
required by the "qgis-plugin-grass", so, I uninstalled the last one.

Best regards

Mauricio
>
>>
>> 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
>>>
>>>
>>
>
> --
> 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
>



-- 
hzambran

Linux user  #454569 -- Ubuntu user #17469