sf: how to get the external ring only from a multipolygon class

Question

I have a data.table with a sfc_multipolygon geometry column. Some rows contain an polygon, others a multi polygon. The column was generated by merging polygons using st_union, which seems to generate some holes. However, I need only the external ring of each polygon.

If I run this:

tnp_c$geometry2 <- lapply(tnp_c$geometry, function(x) {
  if (any(class(x)=="MULTIPOLYGON")) 
    y <- st_polygon(list(x[[1]][[1]]))
  else 
    y <- st_polygon(list(x[[1]]))
  y
})

I can obtain the external polygons, but the result is still a list:

> str(tnp_c, max.level=1)
Classes ‘data.table’ and 'data.frame':  80 obs. of  6 variables:
 $ X        : num  -82.7 -82 -83.6 -83.9 -82.2 ...
 $ Y        : num  46.4 46.8 46.5 46.6 45.8 ...
 $ boq      : logi  FALSE FALSE FALSE FALSE FALSE FALSE ...
 $ geometry :sfc_MULTIPOLYGON of length 80; first list element: List of 8
  ..- attr(*, "class")= chr [1:3] "XY" "MULTIPOLYGON" "sfg"
 $ geometry2:List of 80
 - attr(*, ".internal.selfref")=<externalptr>

Is there a way to force a list to be an sfc_MULTIPOLYGON? Or more generally, is there a simpler way to do this? Thanks in advance for any help!

Carlos Alberto

Does this answer your question? [Removing holes from polygons in R sf](https://stackoverflow.com/questions/52654701/removing-holes-from-polygons-in-r-sf) — Jindra Lacko, May 26 '23 at 06:59
I believe the "getting external ring only" can be reformulated as "removing internal holes" which is a common task often approached via means of `nngeo::st_remove_holes()` — Jindra Lacko, May 26 '23 at 07:01
For list to sfc, try `tnp_c$geometry2 <- st_sfc(tnp_c$geometry2)` — margusl, May 26 '23 at 07:21

sf: how to get the external ring only from a multipolygon class

0 Answers0