[R-sig-Geo] Cleaning small spatial polygons

[Eduardo Diez]

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

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
>>>
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>>>
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>>>
>> --
>> 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
>>
>>
>

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