Graticules in geom_sf

[Nowosad]

Graticules works well in the first example - world1 but there are some problems with the second one - world2:

library(tidyverse)                                                              
library(sf)                                                                     
library(maps)                                                                   
                                                                                
world1 <- st_as_sf(map('world', plot = FALSE, fill = TRUE)) %>%                 
st_transform(., "+proj=laea +y_0=0 +lon_0=155 +lat_0=-90 +ellps=WGS84 +no_defs")
                                                                                
ggplot() + geom_sf(data = world1)                                               

                                                                                
                                                                                
world2 <- st_as_sf(map('world', plot = FALSE, fill = TRUE)) %>%                 
st_transform(., "+proj=laea +y_0=0 +lon_0=-77 +lat_0=39 +ellps=WGS84 +no_defs") 
                                                                                
ggplot() + geom_sf(data = world2)                                               

@edzer should I leave it here or post it as the ggplot2 issue?

[edzer]

Here; it's a problem with st_graticule:

st_graticule() %>% 
  st_transform("+proj=laea +y_0=0 +lon_0=-77 +lat_0=39 +ellps=WGS84 +no_defs") %>% 
  st_geometry %>% 
  plot

[edzer]

Increasing ndiscr in the call to st_graticule helps:

st_graticule(ndiscr=1000) %>% st_transform("+proj=laea +y_0=0 +lon_0=-77 +lat_0=39 +ellps=WGS84 +no_defs") %>% st_geometry %>% plot

but how can we pass this such that it happens inside geom_sf? Maybe geom_sf should accept an ndiscr argument? Yes, this needs work on the ggplot2 side.

[edzer]

With the commit to sf and PR to ggplot2 referenced above, with

ggplot() + geom_sf(data = world2)+ coord_sf(ndiscr=1000)

I now get

[Nowosad]

Thanks @edzer, great work as always!
I have one related question. Why this argument (ndiscr) is added to coord_sf()? I would, naively, thought that coord_sf() is used to set a CRS and not to change any graphical outputs (such as graticules).

[edzer]

Other parameters passed to st_graticule (crs, datum) are also passed on to coord_sf, but I don't master the gg logic yet. @hadley, do you think ndiscr should be a parameter to coord_sf, as in this PR, or rather to geom_sf?

[hadley]

There is one set of graticules and they are equivalent to the grid lines in Cartesian coordinations. I think it's pretty clear that the arguments that control them should belong to coord_sf()

[tsamsonov]

@edzer To continue a discussion on passing the parameters of st_graticule() to coord_sf(): is there any way to control the lat and lon values for automatically generated graticule in ggplot? Sometimes I am not satisified with the default positions of meridians and parallels, like in this plot, where I want the latitude step to be 10 degrees, not 5:

[edzer]

did you try this?

ggplot() + ... + scale_y_continuous(breaks = c(40,50,60))

[tsamsonov]

Thanks, it works! Much easier than I expected.

[edzer]

Same thing here!

[Nowosad]

There is another small thing with graticules.
The only difference between figures is that the first one has +lat_0=40 and the second one - +lat_0=39.
However, only the first one has that unusual line of latitude:

library(sf)
#> Linking to GEOS 3.5.0, GDAL 2.1.2, proj.4 4.9.2

st_graticule(ndiscr = 1000) %>% st_transform("+proj=laea +x_0=0 +y_0=0 +lon_0=-74 +lat_0=40 +ellps=WGS84 +no_defs") %>% st_geometry %>% plot

st_graticule(ndiscr = 1000) %>% st_transform("+proj=laea +x_0=0 +y_0=0 +lon_0=-74 +lat_0=39 +ellps=WGS84 +no_defs") %>% st_geometry %>% plot

Created on 2018-01-19 by the reprex
package (v0.1.1.9000).

[edzer]

It goes away if you do e.g.

st_graticule(ndiscr = 10000) %>% st_transform("+proj=laea +x_0=0 +y_0=0 +lon_0=-74 +lat_0=40.1 +ellps=WGS84 +no_defs") %>% st_geometry %>% plot

that is, change lat_0 away from 40 and increase ndiscr. (The less you move lat_0, the more you need to increase ndiscr).

I suspect this is not an issue with sf, but rather with approximating graticules with linestrings for particular projections with regular discretization of angles instead of focusing on where we need many points. It's very unlikely that I will work on solving this in the near future, but if someone would bring up a PR doing this I'd be happy to review.

I recall that @hadley pointed me once to a website demonstrating javascript code that did dynamic noding of graticules, but forgot where it was.