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@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
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New Zealand
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
