Polygons with getmap() and rnaturalearth data

[andrew-MET]

When trying to extract polygons using rnaturalearth data as the map database, an error is thrown. It appears to be the function maps::map.clip.poly() that is throwing the error.

library(meteogrid)
library(rnaturalearth)
dom <- Make.domain(clonlat = c(17.5, 53), nxny = 100, dxdy = 50000)

# paths work fine
map <- getmap(dom, map.database = countries110)
plot(map, type = "l")

# Polygons throw an error
map <- getmap(dom, map.database = countries110, fill = TRUE)
#> Error in rep(nam, times = dd$npoly): invalid 'times' argument

^{Created on 2025-09-26 with reprex v2.1.1}

[adeckmyn]

I don't think this can be solved easily. countries110 is stored as polygons, not as single lines that are combined into a polygon, like the default world map. Having the maps package work with other input formats was always a bit of a hack anyway and only works by turning the data into a simple set of lines. The polygon clipping is pretty complicated and really needs the internal polygon structure to work correctly. Just dropping all points outside the domain is easy, but knowing how to connect the end points really requires deeper knowledge. Without a full rewrite of the maps code (which is out of the question), I don't see a way to fix this. Unless I use other map and clipping tools (so basically drop maps). Not a bad idea in a way, as maps is a legacy package. But it would take some time.

[andrew-MET]

OK. Maybe for these cases the sp package could be used to extract the data and do the cropping (I'm just assuming that's possible!) and we could add it to Suggests.

[adeckmyn]

Yes, sp or one of the other standard packages should be able to do this. I even remember seeing some code.
A small caveat is that to my knowledge these packages typically use a clipping library that is not totally free (only for non-commercial use).
In the maps package I decided that was not really acceptable (as there are many commercial users), which is why I wrote my own clipping code.

[andrew-MET]

Looking at it quickly, I think sf::st_crop() should be fine. I'll update the get_map() code when I get a chance.