[R-sig-Geo] Sf find number of parts in multigeometry

[Tim Appelhans]

As an addition to obrl soil's answer, I have come to like

lengths(st_geometry(x)) - mind the s!

Best
Tim


On 10/23/2018 06:39 AM, obrl soil wrote:
> Hi Martin,
>
> for a multipolygon geometry column, you're working with a list of
> lists of lists of matrices, so you can iterate over the geometry
> column like
>
>      sapply(st_geometry(x), length)
>
> (or purrr::map_int(), perhaps) to report number of polygon components.
> To subset the first polygon component and drop the rest,
>
>      st_geometry(x) <- st_sfc(lapply(st_geometry(x), function(y)
> st_polygon(y[[1]])), crs = 4326) # or whatever crs you're using
>
> I would first run st_buffer(x, dist = 0L) and/or
> lwgeom::st_make_valid() to ensure the geometries are properly
> structured, although this can't solve every problem. Note that AFAIK
> there's no inherent sorting to multipolygon components, although you
> could probably reorder them by area or number of vertices before
> subsetting if you were really keen.
>
> cheers
> L
> On Tue, Oct 23, 2018 at 12:32 PM Martin Tomko <tomkom using unimelb.edu.au> wrote:
>> I am looking for an equivalent to Postgis ST_NumGeometries https://postgis.net/docs/ST_NumGeometries.html
>> I have multipolygons in an sf df, where most of them are likely to be single part. I want to identify those that are not single part, and possibly retain only the largest polygon part, and cast all into Polygon.
>> Any advice is appreciated, thanks!
>>
>> Martin
>>
>>
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>>
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