layoutsnorth-arrowprint-composerqgissize;
I'm trying to fix the size of my north arrow with 20mm x 20mm. My problem is when my map has a rotation, the arrow is smaller (due to the inner box).
So I'm trying to set the size of my box with:
20*(cos(@map_rotation)+sin(@map_rotation))
But the variable @map_rotation is not available here?
Anyone has a idea to have a north arrow which has the same size, even when the map has a rotation?
There's an option under Item Properties to do this, but it's only available for an Image item, not an Arrow, which may be why you're not seeing it. You need to add your north arrow from an image (Layout -> Add Image). Once you do, there's a checkbox under Image rotation to sync the rotation with your map:
I found this paper from the Journal of Statistical Software (https://www.jstatsoft.org/article/view/v019c01/v19c01.pdf)
It provides the following function:
northarrow <- function(loc,size,bearing=0,cols,cex=1,...) {
# checking arguments
if(missing(loc)) stop("loc is missing")
if(missing(size)) stop("size is missing")
# default colors are white and black
if(missing(cols)) cols <- rep(c("white","black"),8)
# calculating coordinates of polygons
radii <- rep(size/c(1,4,2,4),4)
x <- radii[(0:15)+1]*cos((0:15)*pi/8+bearing)+loc[1]
y <- radii[(0:15)+1]*sin((0:15)*pi/8+bearing)+loc[2]
# drawing polygons
for (i in 1:15) {
x1 <- c(x[i],x[i+1],loc[1])
y1 <- c(y[i],y[i+1],loc[2])
polygon(x1,y1,col=cols[i])
}
# drawing the last polygon
polygon(c(x[16],x[1],loc[1]),c(y[16],y[1],loc[2]),col=cols[16])
# drawing letters
b <- c("E","N","W","S")
for (i in 0:3) text((size+par("cxy")[1])*cos(bearing+i*pi/2)+loc[1],
(size+par("cxy")[2])*sin(bearing+i*pi/2)+loc[2],b[i+1],
cex=cex)
}
I got it to work doing this:
library(GISTools)
data(newhaven)
plot(blocks)
xy = c(530000,160000)#use locator() to get the x,y values for arrow placement
northarrow(loc = xy, size = 10000)#finding the correct size value is a guessing game
You can fiddle with the polygon commands, 
but this one looks pretty nice.
--Also there is another arrow available in the prettymapr package.
Cheers, Lewis
Best Answer
You can compute a (more complex) formula for the rotation of the picture but just few items below, you also have
rotationonly, that rotate the North arrow object, the box / rectangle is not anymore aligned with the layout but your North arrow will always have the same size.To retrieve the map rotation, you have to get its object name and get it with this QGIS expression :
Documentation :