Choropleth plot gets inverted after simplifying geojson with rmapshaper

Question

I want to make a geojson file small/less detailed to speeden up a R Plotly choropleth plot. This is a reproducible plot with the original geojson file:

library(rjson)
library(plotly)

geojson <- rjson::fromJSON(file='https://geodata.ucdavis.edu/gadm/gadm4.1/json/gadm41_DEU_0.json')
#geojson <- geojson2
length(geojson$features)

bl <- sapply(1:length(geojson$features),function(i){geojson$features[i][[1]]$properties$COUNTRY})

for(i in 1:length(geojson$features)){
  geojson$features[[i]]$regio <- geojson$features[i][[1]]$properties$COUNTRY
}

Qkreis <- data.frame(regio = bl,
                     zeroy = rep(0,length(geojson$features))
)
names(Qkreis)[1] <- 'regio'

g <- list(
  fitbounds = "locations",
  projection = list(type = 'mercator'),
  visible = FALSE
)

fig <- plot_ly()
fig <- fig %>% add_trace(
  type="choropleth",
  geojson=geojson,
  locations=Qkreis[,'regio'],
  text=Qkreis[,'regio'],
  z=Qkreis[, 'zeroy'],
  colors = c('white','#ed6a12'),
  showscale=T,
  marker = list(line=list(color='grey',width=0.25)),
  featureidkey = 'regio', 
  geo = 'geo'
)
fig <- fig %>% layout(
  geo = g
)

fig

Then I simplify it with rmapshaper (need to convert it to sf and back to geojson) and put the new object into the 2nd line of the code above. But now the plot gets inverted.

library(geojsonsf)
library(rmapshaper)
library(sf)

geosf <- geojson_sf('https://geodata.ucdavis.edu/gadm/gadm4.1/json/gadm41_DEU_0.json')
substr(geosf, 1, 200)

geosf_simpl <- ms_simplify(geosf, keep = 0.001,
                                keep_shapes = FALSE)

#geoback <- sf_geojson(geosf)
geoback <- sf_geojson(geosf_simpl)
substr(geoback, 1, 200)
geojson2 <- rjson::fromJSON(geoback)
substr(geojson2, 1, 200)

Why, how can I fix it?

I already found out that the problem does not directly happen during simplifying, but at or before choropleth: the normal R plot does not get inverted:

plot(geosf['COUNTRY'])

plot(geosf_simpl['COUNTRY'])
geosf2 <- geojson_sf(geoback)
plot(geosf2['COUNTRY'])

But where and why does the inversion in or before choropleth happen?

Answer 1

solved: the problem is that ms_simplify of rmapshaper lists the geo coordinates clockwise, but choropleth needs them counterclockwise. You need to reverse the sequence of coordinates, e.g. like this:

geosf <- geojson_sf('https://geodata.ucdavis.edu/gadm/gadm4.1/json/gadm41_DEU_0.json')
geosf_simpl <- ms_simplify(geosf, keep = 0.001,
                                keep_shapes = FALSE)
    geosf_simpl$geometry[[1]][1][[1]]
         [,1]    [,2]
 [1,] 10.4548 47.5564
 [2,] 12.1841 47.7011
 [3,] 13.8311 48.7708
 [4,] 12.1996 50.1117
 [5,] 15.0379 51.2440
 [6,] 14.4506 53.2623
 [7,] 13.0325 54.4351
 [8,] 10.0275 54.5504
 [9,]  7.0925 53.5807
[10,]  5.9055 51.0022
[11,]  7.5211 47.6638
[12,] 10.4548 47.5564
    geosf_simpl$geometry[[1]][1][[1]][,1] <- rev(geosf_simpl$geometry[[1]][1][[1]][,1])
    geosf_simpl$geometry[[1]][1][[1]][,2] <- rev(geosf_simpl$geometry[[1]][1][[1]][,2])
    geosf_simpl$geometry[[1]][1][[1]]
        [,1]    [,2]
 [1,] 10.4548 47.5564
 [2,]  7.5211 47.6638
 [3,]  5.9055 51.0022
 [4,]  7.0925 53.5807
 [5,] 10.0275 54.5504
 [6,] 13.0325 54.4351
 [7,] 14.4506 53.2623
 [8,] 15.0379 51.2440
 [9,] 12.1996 50.1117
[10,] 13.8311 48.7708
[11,] 12.1841 47.7011
[12,] 10.4548 47.5564

geoback <- sf_geojson(geosf_simpl)
geojson2 <- rjson::fromJSON(geoback)

Then you get correct choropleth plot: