Why use st_union after st_combine with sf?

Question

The documentation of the st_union function of sf is not clear (for me!) on one point: what are the action and result of the function when applied to an sfc with a single feature, with the parameter by_feature=FALSE (default)? It seems that, according to the text of the documentation, nothing should happen and that the value returned should be the same as the one-feature sfc provided.

I ask this question because it is rather common to find proposals, advice for chaining st_union to st_combine, especially in the context of the use of st_difference (see the sf manual, at geos_binary_ops, or ?st_intersection):

the use of st_difference(SF1,st_union(st_combine(SF2))) is proposed.

Two questions in order to understand:

• Because st_combine returns a single-feature sfc, how could st_union have any effect on such an object?

• Is there any simple case, any example, where st_union, with by_feature=FALSE, applied to a single sfc has any effect?

Extract of the documentation of st_union:

If st_union is called with a single argument, x, and by_feature is FALSE all geometries are unioned together and an sfg or single-geometry sfc object is returned. If by_feature is TRUE each feature geometry is unioned. This can for instance be used to resolve internal boundaries after polygons were combined using st_combine.

Answer 1

I might not have the correct language to exactly explain this, but I just discovered a case where using sf::st_combine before calling sf::st_union made a big difference to me.

I was using dplyr::group_by and dplyr::summarise to group an sf tibble by a certain variable and then union each group to a single sf object:

sf_tibble %>%
  dplyr::group_by(area) %>% 
  dplyr::summarise()

However, what I ended up with was multipolygon geometries, which is fine, but it meant that when I plotted my shapes with text labels, I ended up with multiple text labels for each area. I only wanted a single text label.

Then I tried using st_combine first, then st_union, and this solved my issue. I now only get single text labels for each area. To be explicit:

sf_tibble %>%
  dplyr::group_by(area) %>% 
  dplyr::summarise(across(geometry, ~ sf::st_combine(.)), .groups = "keep") %>% 
  dplyr::summarise(across(geometry, ~ sf::st_union(.)), .groups = "drop")

[It may be that a plain dplyr::summarise() is fine for one of those lines, and I'm not sure how important the .groups bits are.]

(Edit:

sf_tibble %>%
  dplyr::group_by(area) %>% 
  dplyr::summarise(across(geometry, ~ sf::st_combine(.)), .groups = "keep") %>%
  # dplyr::summarise(across(geometry, ~ sf::st_union(.)), .groups = "drop")
  dplyr::summarise()

seems to have the same effect as the code in the previous block.)

I realised that I might need to do this when I read that st_union

can be used to join overlapping and adjacent features

My areas were not all comprised of overlapping and adjacent features - some of them were coastal areas with separate islands. So then I realised that I maybe needed to combine the areas first, before unioning.