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

[Thiago Veloso]

  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.pebe...@uni-muenster.de> wrote:

From: Edzer Pebesma <edzer.pebe...@uni-muenster.de>
Subject: Re: [R-sig-Geo] How to extract coordinates values from a shapefile?
To: r-sig-geo@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...@polygons
    for(i in 1:length(polys)) {
        pp = polys[[...@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"  ))
>> }
>>
>>
> 
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-- 
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.pebe...@wwu.de

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