[R] Interplay rnaturalearth, vwline

[Paul Murrell]

There are a couple of important concepts in play here:

1.
You are drawing the map with the 'graphics' system, but 'vwline' works 
in the 'grid' system.

2.
In the 'grid' system, you can work with lots of different coordinate 
systems.  The default is usually "npc" (which is 0 to 1), but you can 
choose to use others, for example, "native" draws relative to scales on 
the axes (roughly speaking).

If you change the last two lines of your example to the following, it 
might look more like what you expect (?) ...

## Convert the 'graphics' map to a 'grid' equivalent
library(gridGraphics)
grid.echo()
## Navigate to the 'grid' viewport that corresponds to the map
## region
downViewport("graphics-plot-1")
## Draw lines between the corners of the map region
grid.vwline(c(0,1), c(0,1), c(1/1000,1/1000))
grid.vwline(c(0,1), c(1,0), c(1/1000,1/1000))

The next code provides an example of drawing relative to the map 
coordinates (NOTE the use of "native" units) ...

## Navigate to the 'grid' viewport that corresponds to the map
## coordinate system
downViewport("graphics-window-1-1")
## Draw lines relative to the map coordinate system
grid.vwline(mittex + X*band, mittey + Y*band, default.units="native",
             unit(1:9, "mm"), gp=gpar(fill=rgb(1,0,0,.2)))


Hope that helps

Paul


On 14/11/18 6:02 AM, Ferri Leberl wrote:
> Dear All,
> There is something I don't understand fundamentally about handling geocoords with vwlines, as the axample below may illustrate.
> vwlines seems to require values somewhere between 0 and 1 as shares of the plot length and width, but obviously that's not the complete story: Point (1,1) is slightly outside.
> Which spacial framework does vwlines employ?
> How do I have to transform geoocoords to fit them into vwline?
> Thank you in advance!
> Yours, Ferri
> 
> 
> 
> 
> library(rnaturalearth)
> library(grid)
> library(vwline)
> # Install vwline via:
> #library(devtools); install_github("pmur002/vwline/pkg using v0.1")
> #mittex<--59.75#central meridian
> #mittey<--62.316667#central latitude
> mittex<--60.5617;mittey<--62.9832#Whalers Bay
> #mittex<-120;mittey<-0#Sulawesi
> band<-2#halve edge length of the depicted square, in degrees
> map<-ne_countries(scale=10)#the source map
> if(require(sp)){plot(map,ylim=c(mittey-band,mittey+band),xlim=c(mittex-band,mittex+band))}#plot the map
> X<-c(-1,-1,-1,0,0,0,1,1,1)
> Y<-c(-1,0,1,-1,0,1,-1,0,1)
> points(mittex+X*band,mittey+Y*band,col="red")#The map does NOT end at xlim and ylim
> grid.vwline(c(0,1),c(0,1),c(1/1000,1/1000))
> grid.vwline(c(0,1),c(1,0),c(1/1000,1/1000))
> #The vwlines miss the central dot; (1,1) is outside the plot
> 
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul using stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/