[R-sig-Geo] problem with rasterToPolygons x worldclim

[Michael Sumner]

On Wed, 6 Apr 2016 at 23:45 Karla Shikev <karlashikev at gmail.com> wrote:

>
> Hi Michael,
>
> Thanks for the tips. I tried the help for rasterToPolygons, but none of
> the options (e.g. dissolve=TRUE) made any difference. I tried gPolygonize
> and it worked, except for (as you predicted), if the edges meet the raster
> extents - but that definitely was an advance!
>
>
I see the issue, the fun argument to rasterToPolygons is masking out values
from that interval, but it's not setting them all to the same value - so
you still get individual pixel polygons unless you mask on presence in the
interval or not (as a binary):

library(raster)

r<-getData('worldclim', var='bio', res=10)$bio1

e <- extent(-140,-100, 50, 60)

xx <- crop(r,e)
## mask out pixel values first
xx <- xx > 40 & xx < 60

## 576 polygons
slice1<- rasterToPolygons(xx, fun = function(x) {x == 1})
## 1 polygon
slice2 <- rasterToPolygons(xx,  fun = function(x) {x == 1}, dissolve = TRUE)


Still it's not a very nice polygon, if I can I'll try a different way.

Cheers, Mike.




>
> I really appreciate your help and if you just point the way I can try to
> go after what needs to be done, but right now I´m stumped.
>
> best, Karla.
>
> On Mon, Apr 4, 2016 at 5:38 PM, Michael Sumner <mdsumner at gmail.com> wrote:
>
>> Read the help for rasterToPolygons, alternatively try chaining
>> rasterToContour and rgeos::gPolygonize.
>>
>> The latter may need coaxing if your edges meet the raster extents.
>>
>> Cheers, Mike
>>
>> On Tue, 5 Apr 2016, 05:32 Karla Shikev <karlashikev at gmail.com> wrote:
>>
>>> Dear all,
>>>
>>> Here's an issue that is related to the previous one. In the commands
>>> below
>>> I'm trying to make a polygon for all the regions within a range of mean
>>> annual temps. However, rasterToPolygons will draw a square around each
>>> value in the original raster, rather than providing me the actual
>>> polygon,
>>> given that resolution. Any hints? Again, any help will be greatly
>>> appreciated.
>>>
>>> Karla
>>>
>>> _____
>>>
>>> library(raster)
>>>
>>> r<-getData('worldclim', var='bio', res=10)$bio1
>>>
>>> e <- extent(-140,-100, 50, 60)
>>>
>>> xx<-crop(r,e)
>>>
>>> slice1<-rasterToPolygons(xx, fun = function(x) {x > 40 & x < 60})
>>>
>>> plot(slice1)
>>>
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>>>
>>> _______________________________________________
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>>> R-sig-Geo at r-project.org
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>> --
>> Dr. Michael Sumner
>> Software and Database Engineer
>> Australian Antarctic Division
>> 203 Channel Highway
>> Kingston Tasmania 7050 Australia
>>
>>
> --
Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia

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