[R-sig-Geo] Intersect shapefiles

[Roger Bivand]

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

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

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

>
> -----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:
>>
>> 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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-- 
Roger Bivand
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