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After dissolving polygons, some of the internal lines remain. It seem to be a problem common also to ther softwares (ArcGis and Qgis at least). The dissolve result is this:
To dissolve using R, I'm following this recommendation.
Replication code:
library(sf)
library(geobr)
library(dplyr)
cities <- read_municipality(code_muni = "all", year = 2010)
dissolved <- st_make_valid(cities) %>% group_by(code_state) %>% summarize()
plot(dissolved)
How to solve it on R?
As I commented, your problem might be solved by buffering -which means to extend the polygon homogeneously-, dissolve and then use a negative buffer to approximately restore it to its original size; your polygons had remarkably large gaps of around 100 m, usually 0.1 m works for cleaning these kind of polygons.
At the end, change in total area is 0.0007%, which may be negligible.
library(sf)
library(geobr)
library(dplyr)
cities <- read_municipality(code_muni = "all", year = 2010)
# first we buffer
dissolved = cities |>
st_transform(3857) |> st_buffer(100)
# then dissolve
dissolved = dissolved |> group_by(code_state) |> summarise()
# buffer back
dissolved = dissolved |> st_buffer(-100) |>
st_transform(4326)
st_area(dissolved) |> sum()
#8.548294e+12 [m^2]
st_make_valid(cities) |> st_area() |>
sum()
#8.548232e+12 [m^2]
