Let me start by saying I've seen the other thread on a similar question, and even after replicating the nc maps problem they use, I'm still running into a problem with my own code.

I'm trying to create a map with 50km square grids imposed over specific countries. I can generate a map with square grids just fine, but I'm running into problems getting the cellsize argument in st_make_grid() to use sensible metrics.

I understand that I want the maps to use the same projection, and given the units I want one that's using meters. I've checked this repeatedly using st_crs() and it indicates all of the objects are using meters as intended.

This first chunk of code lists the countries I want to include, and lists the CRS code I want to use. I convert each of the three maps returned by gadm() to sf objections, combine the data frames, and apply the CRS code.

library(tidyverse)
library(geodata)

countrylist <- list("DEU", "FRA", "GBR")
wgseqproj <- "EPSG:4087"

tempmap <- countrylist |>
    map(~ gadm(country = .x, level = 0, path = here::here())) |>
    map_dfr(~ st_as_sf(.x)) |>
    sf::st_set_crs(wgseqproj) |>
    sf::st_transform(wgseqproj)

The next chunk of code takes the grid and limits it to the borders of the countries in the list. I take the map from above, make the grid, and use st_intersection() to limit the grid to the country borders. The last line just creates a numeric index for each box in the grid.

  tempgrid <- st_make_grid(tempmap, cellsize = c(0.5, 0.5), square = TRUE, crs = wgseqproj) |> # grid of points
    st_intersection(tempmap) |>
    st_as_sf() |>
    mutate(names = seq(n():1))

ggplot(tempgrid)   geom_sf()