Setting lims alters the shape of geom_sf

[liuyadong]

Describe the bug

Hi, I'm making maps of Russia using geom_sf, but the height/width ratio seems off. Is there a way to maintain the shape as it appears in a global context?

To Reproduce

library(sf)
#> Linking to GEOS 3.13.0, GDAL 3.9.2, PROJ 9.5.0; sf_use_s2() is TRUE
library(rnaturalearth)
library(ggplot2)
russia <- ne_countries(country = "russia", returnclass = "sf")
russia |>
  st_bbox(russia)
#>       xmin       ymin       xmax       ymax 
#> -180.00000   41.15142  180.00000   81.25040

ggplot() +
  geom_sf(data = russia) +
  coord_sf(crs = 4326, expand = F)

ggplot() +
  geom_sf(data = russia) +
  coord_sf(crs = 4326, expand = F) +
  ylim(-90, 90)

^{Created on 2024-10-23 with reprex v2.1.1}

[edzer]

Here are two things you could try:

ortho = st_crs("+proj=ortho +lon_0=90 +lat_0=60")                                                                           |  /tmp/Nvim-R-edzer/Rsource-430273:4:0: unexpected end of input                 
ggplot() +                                                                                                                  |2:   geom_sf(data = st_transform(russia, "+proj=eqc +lon_0=-90") +              
  geom_sf(data = st_transform(russia, ortho))                                                                               |3:   coord_sf(crs = 4326, expand = F)                                           
eqc = st_crs("+proj=eqc +lon_0=90")                                                                                         |  ^                                                                             
ggplot() +                                                                                                                  |> NvimR.chunk(echo=TRUE)                                                        
  geom_sf(data = st_transform(russia, eqc)) 

[liuyadong]

Thanks @edzer Russia is just one example, but more generally, it seems that QGIS can preserve the shape as it appears in a global context. I’m curious if geom_sf offers a similar option without changing the projection.

[edzer]

What exactly do you mean by global context? Could you share the QGIS image?

[liuyadong]

By “global context”, I mean placing a regional map inside a global map. Here is the example of USA.

library(sf)
#> Linking to GEOS 3.13.0, GDAL 3.9.2, PROJ 9.5.0; sf_use_s2() is TRUE
library(rnaturalearth)
library(ggplot2)

world <- ne_countries(returnclass = "sf")
usa <- world |> dplyr::filter(name == "United States of America")

ggplot() +
  geom_sf(data = world)

ggsave("fig-world.png", width = 10, height = 5)

ggplot() +
  geom_sf(data = usa, color = "red", fill = NA) +
  theme_void()+
  theme(plot.background = element_rect(color = 'red'))

ggsave("fig-usa.png", width = 7, height = 5)

If I overlay two figures, the shapes of USA don't match up.

However, in QGIS, the shapes of USA remain consistent regardless of the context.

[liuyadong]

Here is the map of USA alone.

[edzer]

I think that if you add for instance +coord_sf(crs = st_crs("+proj=eqc")) to the two ggplot() commands, you get png files that match when overlayed.

[liuyadong]

@edzer Yes exactly. Could you briefly explain why?
I'm closing this issue.

[edzer]

Could you briefly explain why?

Because for unprojected data a projection needs to be chosen, and a pragmatic decision is to choose equidistant rectangular (map lon/lat linearly to x/y), while choosing an (map) aspect ratio that has unit scale (1 m North = 1 m East) at the center of the map. This aspect ratio is chosen at the center of the map, and the center of the us is different from the center of the world.