Re: [R-sig-Geo] Sf find number of parts in multigeometry
Dear all,
Thank you for the hints – I will test and report any surprises ☺
Martin
From: Michael Sumner
Date: Tuesday, 23 October 2018 at 5:59 pm
To: Martin Tomko
Cc: "r-sig-geo@r-project.org"
Subject: Re: [R-sig-Geo] Sf find number of parts in multigeometry
I see perfectly good answers to this, but can't resist sharing my own approach.
I use gibble::gibble for a summary of parts. See how the multi-part object is
number 4, and has 3 subobjects (subobject will repeat within object for holes).
library(sf)
x <- read_sf(system.file("gpkg/nc.gpkg", package = "sf"))
gibble::gibble(x)
#A tibble: 108 x 5
# nrow ncol type subobject object
#
#127 2 MULTIPOLYGON 1 1
#226 2 MULTIPOLYGON 1 2
#328 2 MULTIPOLYGON 1 3
#426 2 MULTIPOLYGON 1 4
#5 7 2 MULTIPOLYGON 2 4
#6 5 2 MULTIPOLYGON 3 4
#734 2 MULTIPOLYGON 1 5
#...
(I use that for mapping out set-based operations on geometry data, it doesn't
make a huge amount of sense on its own. I suppose a rapply scheme could be
constructed to pluck out things, but you'd also want path extents and sizes and
so forth for greater control).
But, if the biggest area of each multipolygon is what you want, give each a
unique ID, use st_cast to POLYGON, group by parent and slice out the largest.
library(dplyr)
x %>% mutate(ID = row_number()) %>% st_cast("POLYGON") %>%
group_by(ID) %>% arrange(desc(st_area(.))) %>% slice(1) %>% ungroup()
Note that holes within a part might reduce the area logic, but so will details
of the map projection in use and so on. It's helpful to learn the structure of
the underlying geometry lists to craft your own rogue solutions.
HTH
On Tue, Oct 23, 2018, 13:32 Martin Tomko
mailto:tom...@unimelb.edu.au>> wrote:
I am looking for an equivalent to Postgis ST_NumGeometries
https://postgis.net/docs/ST_NumGeometries.html<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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Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia
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Re: [R-sig-Geo] Sf find number of parts in multigeometry
I see perfectly good answers to this, but can't resist sharing my own
approach.
I use gibble::gibble for a summary of parts. See how the multi-part
object is number 4, and has 3 subobjects (subobject will repeat within
object for holes).
library(sf)
x <- read_sf(system.file("gpkg/nc.gpkg", package = "sf"))
gibble::gibble(x)
#A tibble: 108 x 5
# nrow ncol type subobject object
#
#127 2 MULTIPOLYGON 1 1
#226 2 MULTIPOLYGON 1 2
#328 2 MULTIPOLYGON 1 3
#426 2 MULTIPOLYGON 1 4
#5 7 2 MULTIPOLYGON 2 4
#6 5 2 MULTIPOLYGON 3 4
#734 2 MULTIPOLYGON 1 5
#...
(I use that for mapping out set-based operations on geometry data, it
doesn't make a huge amount of sense on its own. I suppose a rapply scheme
could be constructed to pluck out things, but you'd also want path extents
and sizes and so forth for greater control).
But, if the biggest area of each multipolygon is what you want, give each a
unique ID, use st_cast to POLYGON, group by parent and slice out the
largest.
library(dplyr)
x %>% mutate(ID = row_number()) %>% st_cast("POLYGON") %>%
group_by(ID) %>% arrange(desc(st_area(.))) %>% slice(1) %>% ungroup()
Note that holes within a part might reduce the area logic, but so will
details of the map projection in use and so on. It's helpful to learn the
structure of the underlying geometry lists to craft your own rogue
solutions.
HTH
On Tue, Oct 23, 2018, 13:32 Martin Tomko 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
>
>
> [[alternative HTML version deleted]]
>
> ___
> R-sig-Geo mailing list
> R-sig-Geo@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
[[alternative HTML version deleted]]
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Re: [R-sig-Geo] Sf find number of parts in multigeometry
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 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 [[alternative HTML version deleted]] ___ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo ___ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo ___ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo
