st_graticule fails for nzmg

[edzer]

See https://github.com/r-spatial/sf/issues/1047

[edzer]

https://notpeerreviewed.netlify.com/2020/03/mapping-nz-with-an-orthographic-projection/

[notpeerreviewed]

Hi edzer,

I've been looking through the functions that are used by each method my blog post mentioned above. The way I got it to work used the coord_map function within ggplot2, which requires a data.frame rather than an sf object. I haven't been able to figure out what part of the machinery 'under the hood' is responsible for the rendering of the graticules yet, but I am going to keep looking.

[edzer]

Ah, I see. A quick example (of geom_sf?) that did not work well would be useful here.

[notpeerreviewed]

Here's a minimal working example that should demonstrate how coord_sf results in rendering artifacts in the graticules, while coord_map does not.

library(rworldmap)
library(dplyr)
library(ggplot2)
library(mapproj)
library(sf)
library(rnaturalearth)

## MWE for how sf objects create rendering artifacts in orthographic projection
## and how sf objects to not work within coord_map() in ggplot2

# download world data from natural earth
world <- ne_countries(scale = 'small', returnclass = 'sf')


class(world) # returns "sf" "data.frame"

# if we plot this with the standard plotting we get a world map
ggplot() + 
  geom_sf(data=world) 

# if we now add an orthographic projection the horizontal lines become apparent
ortho <- "+proj=ortho +lat_0=-35 +lon_0=170 +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=m +no_defs"
ggplot() + 
  geom_sf(data=world) + 
  coord_sf(crs = ortho)


# now lets try to use the coord_map() function instead of coord_sf
# this results in an error because we need to use coord_sf with geom_sf
ggplot() + 
  geom_sf(data=world) + 
  coord_map("ortho", orientation=c(-35, 175, 0))


# if we want to use the world data with the coord_map() function
# we need to convert it to a dataframe. First we need to convert 
# the sf object to a SpatialPolygonsDataframe so we can fortify it
world_fort <- world %>%
  as("Spatial") %>%
  fortify()

class(world_fort) # returns "data.frame"

ggplot() + 
  geom_polygon(data = world_fort, aes(x = long, y = lat, group = group), fill = "grey60") +
  coord_map("ortho", orientation=c(-35, 175, 0))

> sessionInfo()
R version 3.5.0 (2018-04-23)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18362)

Matrix products: default

locale:
[1] LC_COLLATE=English_New Zealand.1252  LC_CTYPE=English_New Zealand.1252   
[3] LC_MONETARY=English_New Zealand.1252 LC_NUMERIC=C                        
[5] LC_TIME=English_New Zealand.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rnaturalearth_0.1.0 sf_0.8-1            mapproj_1.2.7       maps_3.3.0          ggplot2_3.2.0      
[6] dplyr_0.8.3         rworldmap_1.3-6     sp_1.3-1           

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.1         pillar_1.4.2       compiler_3.5.0     class_7.3-14       tools_3.5.0       
 [6] dotCall64_1.0-0    tibble_2.1.3       gtable_0.2.0       lattice_0.20-35    pkgconfig_2.0.1   
[11] rlang_0.4.0        DBI_1.0.0          rstudioapi_0.7     rgdal_1.4-4        spam_2.5-1        
[16] e1071_1.7-0        withr_2.1.2        rgeos_0.3-28       fields_10.3        classInt_0.4-2    
[21] grid_3.5.0         tidyselect_0.2.5   glue_1.3.1         R6_2.2.2           foreign_0.8-70    
[26] purrr_0.3.2        magrittr_1.5       units_0.6-3        scales_1.0.0       maptools_0.9-9    
[31] assertthat_0.2.0   colorspace_1.3-2   labeling_0.3       KernSmooth_2.23-15 lazyeval_0.2.2    
[36] munsell_0.5.0      crayon_1.3.4      
>

[edzer]

https://github.com/r-spatial/sf/issues/1361