dissolve internal borders of polygons using st_union and group_by

On Thu, 17 Oct 2019, Marta Rufino wrote:

 Hi,

 I am trying to dissolve the internal borders of larger polygons (sf
 object)
 by a grouping variable or by proximity (adjacent)
 and after looking in the web I found several alternatives, but none is
 doing what I wanted.

 ## Reproducible example:
 # World map
 require(rnaturalearth); require(sf)
 world_map <- rnaturalearth::ne_countries(scale = 'small', returnclass =
 c("sf"))

 # World countries:
 world_map
 object.size(world_map)

 # 1st alternative: in this case we have the groups by continent, but the
 country boundaries are still there, although without label:
 (kk <- world_map %>%
  dplyr::group_by(continent) %>%
  summarise())
 object.size(kk)
 kk %>%
  st_transform(crs = 42310) %>%
  ggplot()+
  geom_sf()

Please state all versions:

sessionInfo()
sf_extSoftVersion()

With an updated system, most of your code just does not work for me. You are 
looking for sf::aggregate():

kk <- aggregate(world_map, list(world_map$continent), head, n=1)

plot(st_geometry(kk))

shows that although the country boundaries are largely removed, the 
underlying data are not properly aligned, so slivers remain, some on 
continent boundaries, some as holes in land masses.

Contributions welcome to remove the artefacts.

Using tidyverse really occludes analysis here.

Note that EPSG 42310 simply does not exist in PROJ 6, it is retrievable from:

kk1 <- st_transform(kk, crs = paste0("+proj=merc +lat_ts=0 +lon_0=0",
" +k=1.000000 +x_0=0 +y_0=0 +ellps=GRS80 +datum=WGS84 +units=m"))
plot(st_geometry(kk1))

and should never be used for obvious reasons.

kk2 <- st_transform(kk, crs=3857)
plot(st_geometry(kk2))

is not much better (Web Mercator).

Hope this clarifies,

Roger

 # 2nd alternative : apparently different, but very similar...
 (kk1 <- world_map %>%
  dplyr::group_by(continent) %>%
  summarise(st_union(.)))
 object.size(kk1)
 kk1 %>%
  st_transform(crs = 42310) %>%
  ggplot()+
  geom_sf()

 # 3rd alternative
 (kk2 <- world_map %>%
  dplyr::group_by(continent) %>%
  summarise(geom = st_union(geometry)))
 object.size(kk2)
 kk2 %>%
  st_transform(crs = 42310) %>%
  ggplot()+
  geom_sf()

 # 4th alternative
 (kk3 <- world_map %>%
  dplyr::group_by(continent) %>%
  summarise(geom = st_combine(geometry)))
 object.size(kk3)
 kk3 %>%
  st_transform(crs = 42310) %>%
  ggplot()+
  geom_sf()


 # In all cases the objects produced are actually very similar at a first
 glance, bur in fact they differ on the properties (as reflected by their
 sizes).
 object.size(world_map)
 object.size(kk1)
 object.size(kk2)
 object.size(kk3)
 object.size(kk4)

 # And more importantly, in no case the borders between countries were
 actually dissolved, which was my objective.


 So, the question is how can I get the continents with the country borders
 dissolved?
 Further, could I dissolve all contiguous borders (instead of having a
 grouping variable)- I saw this in a couple of posts but the answers were
 very complex.

 Any help will be greatly appreciated,
 Best wishes,
 M.