[R-sig-Geo] Clipping a Map

[Robert J. Hijmans]

Ravi,
I think you can use the crop function in raster (which calls the rgeos
function gIntersect, but also does the necessary attribute
housekeeping):

library(raster)
x <- crop(camapzip_temp, camap_base)

Robert





On Fri, Nov 22, 2013 at 11:31 AM, Ravi Vaid <rvaid at hotmail.com> wrote:
>
>
>
> Dude, You are going way over my head. I think I lost you in the first sentence. Since this is probably a common task that alot of people have to do, can you provide an example of doing this. That would really help. I have a map of western US (camapzip_temp) and I am clipping the California portion of that. The CA (camap_base) portion is just a single polygon. The US map has zip code polygons and data attributes (zip codes) are attached to this map. I know your time is valuable but when ever you have some free time can you proved the modifications that are necessary to the code below. I would really appreciate it. This might be a silly question but why does the gIntersection function not work in this case. Keep in mind I am really new at this. My original attempt is below,
>
> {
> cz_zip <- gIntersects(camapzip_temp,camap_base, byid=TRUE)
> out <- lapply( which(cz_zip) , function(x){ gIntersection( camapzip_temp[x,] , camap_base ) } )
> keep <- sapply(out, class)
> out <- out[keep == "SpatialPolygons"]
> czout <- SpatialPolygons( lapply( 1:length( out ) , function(i) { Pol <- slot(out[[i]], "polygons")[[1]];   slot(Pol, "ID") <- as.character(i)
>                                                                    Pol}))
> }
>
>
>
>
>
>
>
>> Date: Fri, 22 Nov 2013 19:59:53 +0100
>> From: Roger.Bivand at nhh.no
>> To: rvaid at hotmail.com
>> CC: berman.jesse at gmail.com; r-sig-geo at r-project.org
>> Subject: Re: [R-sig-Geo] Clipping a Map
>>
>> On Fri, 22 Nov 2013, Ravi Vaid wrote:
>>
>> >
>> > Jesse,
>> >
>> > The clipping of the larger map by a smaller region works perfectly, but the resulting map (the clipped version) has the data component of the SP object missing. I can't merge anything if the data component is not there. I have tried directly using the gintersection function to clip the map but that seems to fail (waited more than 2 hours for the program to run).
>> > When I run the command: slotNames(czout) I should get an object of name "data" along with the other 4 components below
>> > but I only get,
>> > I get: [1] "polygons"    "plotOrder"   "bbox"        "proj4string"So, somehow the "data" object is getting lost. I am fairly new to GIS, I just found the code for clipping on the sig-geo list and tried it and it worked, but the "data" object is missing. I know how to merge data but I just can't figure out how to recover the "data" component of the SP object.
>>
>> No, you get a SpatialPolygons object. Obviously, the intersection will be
>> some subset of your clipped input object, but the function cannot lknow or
>> guess what you want. Suppose that you had a variable in the data slot that
>> was a count of some phenomenon - we don't know what proportion of the
>> count to retain if only part of the geometry is returned. This is clearly
>> up to the user to decide. Use the row.names() of the input and output
>> objects to identify which are which - use the accessor method to retrieve
>> the string values
>>
>> row.names(camapzip_temp)
>> row.names(camap_base)
>>
>> and
>>
>> row.names(cz_zip)
>>
>> which are catenated from the input object row.names.
>>
>> Hope this clarifies,
>>
>> Roger
>>
>> PS. Never, ever, assign directly to the data slot - this almost always
>> creates havoc and confusion by sidestepping checking of row.name values.
>> It's a really great way of randomising your data by accident, but
>> otherwise has no good justification.
>>
>> >
>> > Ravi
>> >
>> >
>> >> Date: Fri, 22 Nov 2013 08:36:52 -0800
>> >> From: berman.jesse at gmail.com
>> >> To: r-sig-geo at r-project.org
>> >> Subject: Re: [R-sig-Geo] Clipping a Map
>> >>
>> >> Hi Ravi,
>> >>
>> >> After performing 'gIntersects' try merging this result to your dataframe,
>> >> then select the TRUE values?
>> >>
>> >> Jesse
>> >>
>> >>  cz_zip <- gIntersects(camapzip_temp,camap_base, byid=TRUE)
>> >> camap_base at data<-cbind(camap_base at data, czip)
>> >> camap2<-camap_base[(camap_base at data[,/???/] %in% c("TRUE)),] # Insert your
>> >> column number for czip in place of ???
>> >>
>> >> plot(camap2)
>> >> head(camap2 at data)
>> >>
>> >>
>> >>
>> >> -----
>> >> --------------------------------------------------------
>> >> Jesse D Berman, PhD
>> >> Yale University
>> >> School of Forestry and Environmental Studies
>> >> Post-Doc Fellow
>> >> --
>> >> View this message in context: http://r-sig-geo.2731867.n2.nabble.com/Clipping-a-Map-tp7585156p7585162.html
>> >> Sent from the R-sig-geo mailing list archive at Nabble.com.
>> >>
>> >> _______________________________________________
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>> >> R-sig-Geo at r-project.org
>> >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>> >
>> >
>> >
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>> >
>>
>> --
>> Roger Bivand
>> Department of Economics, NHH Norwegian School of Economics,
>> Helleveien 30, N-5045 Bergen, Norway.
>> voice: +47 55 95 93 55; fax +47 55 95 95 43
>> e-mail: Roger.Bivand at nhh.no
>>
>
>
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