north arrow and scale bar in ggplot2

[Manuel Spínola]

Dear list members,

I am doing spatial plot with ggplot2 (data frame from a raster) and I am wonder if it is possible to include a north arrow and a scale bar in the plot.

Best,

Manuel

--
Manuel Spínola, Ph.D.
Instituto Internacional en Conservación y Manejo de Vida Silvestre
Universidad Nacional
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[David Kahle]

Not build in. :) You could add one pretty easily with annotate() though...

david.

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[Manuel Spínola]

Thank you David.

How I do that?

Best,

Manuel

2011/5/17 David Kahle <david...@gmail.com>

[Osmo Salomaa]

For anyone interested, here are two functions I wrote a while back to
simplify adding a very simple scalebar to a plot.

hscale_segment = function(breaks, ...)
{
y = unique(breaks$y)
stopifnot(length(y) == 1)
dx = max(breaks$x) - min(breaks$x)
dy = 1/30 * dx
hscale = data.frame(ix=min(breaks$x), iy=y, jx=max(breaks$x),
jy=y)
vticks = data.frame(ix=breaks$x, iy=(y - dy), jx=breaks$x, jy=(y +
dy))
df = rbind(hscale, vticks)
return(geom_segment(data=df,
aes(x=ix, xend=jx, y=iy, yend=jy),
...))

}

hscale_text = function(breaks, ...)
{
dx = max(breaks$x) - min(breaks$x)
dy = 2/30 * dx
breaks$y = breaks$y + dy
return(geom_text(data=breaks,
aes(x=x, y=y, label=label),
hjust=0.5,
vjust=0,
...))

}

Then it's a simple matter of defining the break points for the
scalebar and adding layers with the two above functions to your plot.
Defining the breaks obviously depends on your coordinate system, the
below example is for a projected metric coordinate system. (ymin,
xmax) is the lower right hand corner in which the scalebar is being
placed.

scalebreaks = data.frame(x=seq(xmax - 11000, xmax - 1000, 5000),
y=(ymin + 1000),
label=c("0", "5", "10 km"))

plot = (ggplot(...)
+ ...
+ hscale_segment(scalebreaks, size=0.1)
+ hscale_text(scalebreaks, size=2)
+ ...
)

It will look about like this:

0 5 10 km
|--------|--------|

Adding a north arrow should be trivial, assuming you yourself know
which way north is.

[Pet Chiang]

How can I find out my map xmax and ymin?