An R function to download shapefiles

This post is a follow up from my latest Things I Forget post on reading in shapefiles.  That post assumed that you already had access to all the relevant files (e.g. .shp, .shx, .prj, .dbf, etc.).  A task that I routinely need to do is locate shapefiles on a website, grab those files, and read them in.  Instead of having to do this manually I wrote a function a while back to take care of this task.  The function simply requires shape_url, a link to the location of the files, and layer, the name of the shapefile. Currently the layer should not contain the .shp extension. There is also an optional parameter outfile that can be used to create a different name for the downloaded files.

I haven’t spent much time on error handling. For instance this function assumes you already have rgdal and sp installed and loaded. If you try to use this and get an error, let me know in the comments and I’ll try to fix it. Or better yet suggest a change and I’ll throw it in!

So here is the function.

download.shapefile<-function(shape_url,layer,outfile=layer)
{
  #written by: jw hollister
  #Oct 10, 2012

  #set-up/clean-up variables
  if(length(grep("/$",shape_url))==0)
  {
    shape_url<-paste(shape_url,"/",sep="")
  }
  #creates vector of all possible shapefile extensions
  shapefile_ext<-c(".shp",".shx",".dbf",".prj",".sbn",".sbx",
                   ".shp.xml",".fbn",".fbx",".ain",".aih",".ixs",
                   ".mxs",".atx",".cpg")

  #Check which shapefile files exist
  if(require(RCurl))
  {
    xurl<-getURL(shape_url)
    xlogic<-NULL
    for(i in paste(layer,shapefile_ext,sep=""))
    {
      xlogic<-c(xlogic,grepl(i,xurl))
    }

    #Set-up list of shapefiles to download
    shapefiles<-paste(shape_url,layer,shapefile_ext,sep="")[xlogic]
    #Set-up output file names
    outfiles<-paste(outfile,shapefile_ext,sep="")[xlogic]   }
    #Download all shapefiles
    if(sum(xlogic)>0)
    {
      for(i in 1:length(shapefiles))
      {
        download.file(shapefiles[i],outfiles[i],
                      method="auto",mode="wb")
      }
      } else
      {
      stop("An Error has occured with the input URL
            or name of shapefile")
    }
}

And now to prove it works I can do something like the following:

#Download the NH State Boundaries
download.shapefile("ftp://ftp.granit.sr.unh.edu/pub/GRANIT_Data/Vector_Data/Administrative_and_Political_Boundaries/d-nhsenatedists/2012",
                   "NHSenateDists2012")
#Read shapefiles in SpatialPolygonsDataFrame
NHBnd<-readOGR(".","NHSenateDists2012")
#Plot it
plot(NHBnd)

New Hampshire State Boundary and Senate District

Lastly, there are some other approaches for tackling related problems listed below.

Related articles

• Batch Downloading Zipped Shapefiles with R (r-bloggers.com)
• Loading Multiple Shapefiles to the R-Console Simultaneously (r-bloggers.com)

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Things I Forget: Reading a Shapefile in R with readOGR

One of the more common ways that I read vector data into R is via shapefiles.  I tend to use these partly becuase of my own sordid past with Arc/INFO, ArcView and ArcGIS and partly due to their ubiquity.  In any event I have found the R package, rgdal, indespensible for this.  One of the workhorse functions for pulling in vector data is readOGR().  It has two required parameters dsn and layer. The part I never remember is how these relate to shapefiles. There is nothing especially tricky about it, I just tend to forget what the dsn is and what the layer is. In short the dsn is the directory (without a trailing backslash) and the layer is the shapefile name without the .shp.

So, here’s the actual code so I don’t have to look it up again.

If the shapefile you are reading is in your current working directory the dsn refers simply to that directory. So all you need is simply a “.”. The layer is the name of shapefile without an extension. So it would look something like:

myShapeInR<-readOGR(".","myShapeFile")

Now, if that file resides elsewhere, the trick is to remember what the heck dsn refers to. Again it is simply the directory where the shapefile resides. So if I had a shapefile in a place like C:/data I would use the command like:

myShapeInR<-readOGR("C:/data","myShapeFile")

There, it is really quite simple, yet I always mess it up. Not any more.

I hope.