Dissolve boundaries only of sf overlapping polygons

[szauli]

I have some sf polygons and would like to dissolve the boundaries of overlapping polygons. In other words I would like to have as a results only two polygons.

I tried with st_union but I didn't find options to achieve my objective.
Any suggestions? Stefano

[technocrat]

This S/O post is probably the most direct solution, assuming the boundaries don't need to be preserved or if working with dual sf objects is not an objection.

[jlacko]

When dissolving polygons you need a variable to dissolve by; for a more detailed walktru consider this earlier blog post Merging Geometry of {sf} Objects in R · Jindra Lacko

In your case - when one of your desired polygon groups seems to have only a single member - you could go by name of the isolated island polygon. Consider this piece of code built on the well known & much loved nc.shp shapefile:

library(sf)
library(dplyr)


shape <- st_read(system.file("shape/nc.shp", package="sf")) # included with sf package

merged_shape <- shape %>% 
  mutate(is_cnty_ashe = ifelse(NAME == "Ashe", TRUE, FALSE)) %>% 
  group_by(is_cnty_ashe) %>% 
  summarise()

plot(merged_shape)

What it does is that it dissolves 100 NC counties to two groups - one containing County Ashe, and the other for the rest of NC. Then it uses this grouping to dissolve boundaries using dplyr::summarise() call.