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Intersect shapefiles

11 messages · Loïc Dutrieux, A. Marcia BARBOSA, Chris English +4 more

Original
John Wasige
# 
?Dear all,

I have three shapefiles that I want to intersect. Any body with an idea on
how to do the intersection? Shapefiles below;
library(rgdal)
library(rgeos)
library(maptools)
library(sp)
###shape1
Rain_poly_merge <- readOGR(dsn="G:/EPIC_CUP/Rain_poly_merge.shp",
layer="Rain_poly_merge")
Rain_poly_merge <- spTransform(Rain_poly_merge, CRS("+init=epsg:4326"))

###shape2
Rain_poly_merge <- readOGR(dsn="G:/EPIC_CUP/yield_maize_average2002.shp",
layer="Rain_poly_merge")
Rain_poly_merge <- spTransform(Rain_poly_merge, CRS("+init=epsg:4326"))

###shape3
Rain_poly_merge <- readOGR(dsn="G:/EPIC_CUP/yield_maize_average2008.shp",
layer="Rain_poly_merge")
Rain_poly_merge <- spTransform(Rain_poly_merge, CRS("+init=epsg:4326"))
Loïc Dutrieux
# 
Hi,

Have you tried gIntersection from the rgeos package.

Given 3 SpatialPolygon* objects you can probably do something like:

sp12 <- gIntersection(sp1, sp2)
sp123 <- gIntersection(sp12, sp3)

Cheers,
Lo?c
On 02/18/2016 10:25 AM, John Wasige wrote:
A. Marcia BARBOSA
# 
Check out also function joinPolys in the PBSmapping package
(http://www.inside-r.org/packages/cran/pbsmapping/docs/joinPolys)


- - -
A. M?rcia BARBOSA, post-doctoral fellow
CIBIO/InBIO - University of ?vora (Portugal)
http://modtools.wordpress.com/barbosa



2016-02-18 10:04 GMT+00:00 Lo?c Dutrieux <loic.dutrieux at wur.nl>:

  
  


  


      

    

    

      
      

        


  

        

      Chris English
    

    

      
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John,

As Loic suggests, rgeos::gIntersection will get you there. Look closely at
his naming of objects and your example code using spTransform. With his
approach, each object has its own name; whereas your approach overwrites
Rain_poly_merge the second and third time and the final object (third) is
the 2008 maize yield. So read your shapefiles into differently named
objects and use Loic's approach.

Chris

On Thu, Feb 18, 2016 at 1:58 PM, A. Marcia BARBOSA <barbosa at uevora.pt>
wrote:

            

      
      
      
    

        

      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      John Wasige
    

    

      
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Thanks so much

On Thu, Feb 18, 2016 at 3:23 PM, chris english <

        
englishchristophera at gmail.com> wrote:

        

            

      
      
      
    

        

      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Worrall, James -FS
    

    

      
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I would have thought gUnaryUnion, but then I can't understand from the documentation what the difference is between that and gIntersection.

-----Original Message-----
From: R-sig-Geo [mailto:r-sig-geo-bounces at r-project.org] On Behalf Of chris english
Sent: Thursday, February 18, 2016 7:24 AM

John,

As Loic suggests, rgeos::gIntersection will get you there. Look closely at his naming of objects and your example code using spTransform. With his approach, each object has its own name; whereas your approach overwrites Rain_poly_merge the second and third time and the final object (third) is the 2008 maize yield. So read your shapefiles into differently named objects and use Loic's approach.

Chris





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      Alex Mandel
    

    

      
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Union creates a merged shape including all parts of input objects.

Intersection creates a shape of the overlapping portions of input objects.

I'm not clear on the goal from the initial email so I don't know which
is better in this case.

Enjoy,
Alex

        
On 02/18/2016 07:55 AM, Worrall, James -FS wrote:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Worrall, James -FS
    

    

      
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Such a clear and simple explanation - thank you.  I wish the function documentation was written like that.

-----Original Message-----
From: Alex Mandel [mailto:tech_dev at wildintellect.com] 
Sent: Thursday, February 18, 2016 9:59 AM
To: Worrall, James -FS <jworrall at fs.fed.us>; chris english <englishchristophera at gmail.com>; A. Marcia BARBOSA <barbosa at uevora.pt>
Cc: Help R-Sig_Geo <r-sig-geo at r-project.org>
Subject: Re: [R-sig-Geo] Intersect shapefiles

Union creates a merged shape including all parts of input objects.

Intersection creates a shape of the overlapping portions of input objects.

I'm not clear on the goal from the initial email so I don't know which is better in this case.

Enjoy,
Alex

        
On 02/18/2016 07:55 AM, Worrall, James -FS wrote:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Chris English
    

    

      
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And so for John 's completion we should ask: Union or intersection? I
assume union but of course could be wrong. What i especially like about the
sp and rgeos documentation is that if you take the time to run the examples
in the documentation you just might see your use case. Or, rather likely.
Chris

        
On Feb 18, 2016 8:11 PM, "Worrall, James -FS" <jworrall at fs.fed.us> wrote:

        

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      Roger Bivand
    

    

      
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On Thu, 18 Feb 2016, Worrall, James -FS wrote:

        

            

      
      
      
    

        
Please feel free to contribute; the description section shall be short, 
what you seem to want is in the details a little further down the page:

"Returns an sp object with intersecting geometries merged. If geometries 
do not intersect then both are returned as distinct subgeometries."

Note that gUnion is a binary operator (union of two objects) while 
gUnaryUnion is a unary operator with an id argument to map input 
subgeometries to output subgeometries.

I guess this is easier to grasp than the JTS original describing the 
binary union operator: "Computes a Geometry representing all the points in 
this Geometry and other" at

http://www.vividsolutions.com/jts/javadoc/com/vividsolutions/jts/geom/Geometry.html#union%28com.vividsolutions.jts.geom.Geometry%29

Roger

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Worrall, James -FS
    

    

      
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I think this was the confusing part to me, but I am easily confused.  If two polygons overlap, in my mind the two polygons are intersecting geometries, and thus gIntersection should return the two polygons, in their entirety, merged.  Of course, this is the same as gUnion, so that doesn't make sense.

I made some simple polygons and tested to make sure gIntersection and gUnion behave as I now think they do.

Maybe it would be clearer to say something like, "Returns an sp object with intersecting (overlapping) portions of intersecting geometries merged.  Non-intersecting portions of intersecting geometries are removed.  If geometries do not intersect then both are returned as distinct subgeometries."

            

      
      
      
    

        
Yes indeed!




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