[R-sig-Geo] How to extract coordinates values from a shapefile?

[Edzer Pebesma]

long = q[,1]
lat  = q[,2]

On 06/14/2010 05:05 AM, Thiago Veloso wrote:
>   This function worked like a charm, but I can't individually invoke the columns which contain the coordinates of variable "q".
>> names(q)NULL
>   For example, I need to plot the coordinates using polygon function, whose help tells us that "‘polygon’ draws the polygons whose vertices are given in ‘x’ and ‘y’".
>   So, I'd like to be able to refer to lon and lat columns of variable "q" as "q$lon" and "q$lat".
>   Is it possible??
>   Best wishes,
>   Thiago.
> 
> --- On Wed, 9/6/10, Edzer Pebesma <edzer.pebesma at uni-muenster.de> wrote:
> 
> From: Edzer Pebesma <edzer.pebesma at uni-muenster.de>
> Subject: Re: [R-sig-Geo] How to extract coordinates values from a shapefile?
> To: r-sig-geo at stat.math.ethz.ch
> Date: Wednesday, 9 June, 2010, 17:24
> 
> The example provided by Matt assumes that each polygon consists of a
> single ring, and doesn't have islands, lakes etc. The function below
> pasts all coordinates to a single 2-column matrix. For clarity's sake, I
> avoided nested sapply's.
> 
> library(maptools)
> xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
>   IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
> allcoordinates = function(x) {
>     ret = NULL
>     polys = x at polygons
>     for(i in 1:length(polys)) {
>         pp = polys[[i]]@Polygons
>         for (j in 1:length(pp))
>             ret = rbind(ret, coordinates(pp[[j]]))
>     }
>     ret
> }
> q = allcoordinates(xx)
> 
> # check:
> plot(xx)
> lines(q, col='blue')
> 
> 
> On 06/09/2010 09:58 PM, Matt Beard wrote:
>>>
>>> library(maptools)
>>>
>>> # A simple way to print out a list of coordinates for each polygon in your
>>> shapefile:
>>>
>>> # Path and filename of polygon shapefile
>>> testfile <- '/media/PKBACK# 001/FNR210/QGISLab/habitats/habitats.shp'
>>>
>>> # Read in polygon shapefile using handy maptools function
>>> test <- readShapePoly(testfile)
>>>
>>> # Extract the list of Polygons objects
>>> polys <- slot(test,"polygons")
>>>
>>> # Within each Polygons object
>>> #    Extract the Polygon list (assuming just one per Polygons)
>>> #    And Extract the coordinates from each Polygon
>>> for (i in 1:length(polys)) {
>>>     print(paste("Polygon #",i))
>>>     print(slot(slot(polys[[i]],"Polygons")[[1]],"coords"  ))
>>> }
>>>
>>>
>>
>>     [[alternative HTML version deleted]]
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> 

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de