[R-sig-Geo] Cleaning small spatial polygons

[Roger Bivand]

On Wed, 4 Nov 2015, Eduardo Diez wrote:

> Is there any way of doing this or should i forget it and go on using GRASS
> through rgrass7?

I suggest working with Emmanuel Blondel (cleangeo maintainer) to extend 
cleangeo (also suggested in an earlier thread today, with apologies to 
Emmanuel for picking on him!). That package already uses rgeos, and is a 
logical place to put GRASS v.clean-like functionality (maybe even 
encapsulating using GRASs via rgrass7 and a throwaway location).

At the moment it is messy, though some rgeos internals do do something 
like this, but it isn't exposed to users.

Roger

>
> Thanks
>
> 2015-10-19 15:03 GMT-03:00 Eduardo Diez <eduardodiez at gmx.com>:
>
>> Ok. So here's a link to a zip file that contains two shapefiles:
>>  - pol_to_be_cleaned: the layer from which i'd like to remove small
>> polygons
>>  - pol_cleaned: the layer cleaned with the function v.clean rmarea
>>
>> http://1drv.ms/1GmRWS7
>>
>> The threshold i used for cleaning was 3000 (meaning 3000 squared meters).
>>
>> Although i do project it before sending it to GRASS, according to the
>> official help page it should be able to handle it:
>> "Threshold must always be in square meters, also for latitude-longitude
>> locations or locations with units other than meters"
>>
>> Thanks
>>
>> 2015-10-19 8:28 GMT-03:00 Roger Bivand <Roger.Bivand at nhh.no>:
>>
>>> On Fri, 16 Oct 2015, Eduardo Diez wrote:
>>>
>>> Dear list,
>>>> I'm willing to know if any knows a way of performing tha same thing i'm
>>>> doing through rgrass7 with GRASS when I execute the function v.clean with
>>>> "rmarea" as the tool argument. That is:
>>>>
>>>> "The rmarea tool removes all areas <= thresh. The longest boundary with
>>>> an
>>>> adjacent area is removed or all boundaries if there is no adjacent area.
>>>> Area categories are not combined when a small area is merged with a
>>>> larger
>>>> area."
>>>>
>>>> Basically i have raster of zones within a field. I convert it to
>>>> SpatialPolygonsDataFrame and in order to leave only the more
>>>> important/meaningful ones i remove the small/sliver with this tool. In
>>>> general it works fine but having to call an external software with a
>>>> specific version makes the script less portable and you have to be
>>>> careful
>>>> with updates and such. Also you have to write rasters and shapefiles back
>>>> and forth as GRASS can't work with in-memory objects.
>>>>
>>>
>>> Could you please provide an example of a built-in or contributed data set
>>> (URL, not attachment) with the slivers you mention, so that we know that we
>>> are addressing your problem? I don't think that:
>>>
>>> https://cran.r-project.org/web/packages/cleangeo/index.html
>>>
>>> does this, as it seems to try to repair broken geometries.
>>>
>>> Also note that you need to specify that the area threshold is in a square
>>> planar metric - dropping slivers in unprojected geometries may be more
>>> complicated.
>>>
>>> Roger
>>>
>>>
>>>> Does someone know a way of doing this in plain R?
>>>>
>>>> Thanks
>>>>
>>>>         [[alternative HTML version deleted]]
>>>>
>>>> _______________________________________________
>>>> R-sig-Geo mailing list
>>>> R-sig-Geo at r-project.org
>>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>>
>>>>
>>> --
>>> Roger Bivand
>>> Department of Economics, Norwegian School of Economics,
>>> Helleveien 30, N-5045 Bergen, Norway.
>>> voice: +47 55 95 93 55; fax +47 55 95 91 00
>>> e-mail: Roger.Bivand at nhh.no
>>>
>>>
>>
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no