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Buffer regions / owin to spatial object

3 messages · Giorgio Guzzetta, Medina, Hernan, Roger Bivand

Original
Giorgio Guzzetta
# 
Hello everybody.

I have a set of points and I want to calculate a buffer region within a
given distance from these points without using packages requiring other 
software (such as spgrass6). I have used function disc() for each
point and then union.owin() (both from spatstat) to merge the single
buffers.
The result is an object of class owin, but I would need to obtain an 
object representing the polygons enclosing this buffer.

1) Are there smarter ways to obtain the buffer region (the union.owin is
very slow when it comes to hundreds of points, as it must be applied to 
only two owin at a time)? It seems there isn't for buffers around 
polygons (as Adrian Baddeley wrote in a recent thread).

2) If not, how do I convert the owin object into a Spatial object? There 
are many functions doing the inverse, but I couldn't find any doing this
conversion.

Sorry if the questions are naive... I'm an R beginner :)

Thanks,
Giorgio Guzzetta
Medina, Hernan
# 
I have a question similar to Giorgio's.  I have a set of points that
cover a large region that could be projected as a parallelogram
including Canada, Greenland, the USA, and Mexico, as well as portions of
the Artic, Atlantic and Pacific oceans and the Great Lakes.  I used R
and the maps and mapdata packages to obtain the set points on land with
the in.polygon internal function.  I would like, however, to have a
buffer region of roughly 10 miles (16 km) around the coastal areas.
This seems similar to what Giorgio is trying to accomplish.

Hernan Medina


-----Original Message-----
From: r-sig-geo-bounces at stat.math.ethz.ch
[mailto:r-sig-geo-bounces at stat.math.ethz.ch] On Behalf Of Giorgio
Guzzetta
Sent: Tuesday, November 27, 2007 8:52 AM
To: r-sig-geo at stat.math.ethz.ch
Subject: [R-sig-Geo] Buffer regions / owin to spatial object

Hello everybody.

I have a set of points and I want to calculate a buffer region within a
given distance from these points without using packages requiring other 
software (such as spgrass6). I have used function disc() for each
point and then union.owin() (both from spatstat) to merge the single
buffers.
The result is an object of class owin, but I would need to obtain an 
object representing the polygons enclosing this buffer.

1) Are there smarter ways to obtain the buffer region (the union.owin is
very slow when it comes to hundreds of points, as it must be applied to 
only two owin at a time)? It seems there isn't for buffers around 
polygons (as Adrian Baddeley wrote in a recent thread).

2) If not, how do I convert the owin object into a Spatial object? There

are many functions doing the inverse, but I couldn't find any doing this
conversion.

Sorry if the questions are naive... I'm an R beginner :)

Thanks,
Giorgio Guzzetta

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1 day later
Roger Bivand
#
On Tue, 27 Nov 2007, Giorgio Guzzetta wrote:

            

      
      
      
    

        
If you can move to raster, you can use erode and dilate methods for owin 
objects to buffer in and buffer out - these are based on "im" objects. It 
will not be quite the same, but the errors for moderate resolution 
probably will not be greater than the inaccuracies in polygon boundaries 
(where curves are represented as line segments).

library(maptools)
library(spatstat)
set.seed(1)
pts <- runifpoint(20)
grd <- GridTopology(c(0.005, 0.005), c(0.01, 0.01), c(100, 100))
ptsi <- overlay(SpatialGrid(grd), as(pts, "SpatialPoints"))
SGDF <- SpatialGridDataFrame(grd, data=data.frame(pts=rep(NA, 10000)))
SGDF$pts[ptsi] <- 1
pts_im <- as(SGDF, "im")
pts_owin <- as.owin(pts_im)
plot(pts_owin)
plot(pts, add=TRUE)
pts_owin_0.03 <- dilate.owin(pts_owin, r=0.03)
plot(pts_owin_0.03)
plot(pts, add=TRUE)

gets part of the way there, but most likely better routes exist.

            

      
      
      
    

        
You can get from "im" to SpatialGridDataFrame and from "ppp" fo several sp 
classes. So if you go with raster, you can retrieve the buffered objects.

Hope this helps,

Roger