fields (version 5.02)

arrow.plot: Adds arrows to a plot

Description

Adds arrows at specified points where the arrow lengths are scaled to fit on the plot in a reasonable manner. A classic use of this function is to depict a vector field. At each point (x,y) we have a vector with components (u,v). Like the arrows function this adds arrows to an existing plot.

Usage

arrow.plot(a1, a2, u = NA, v = NA, arrow.ex = 0.05,  
     xpd = TRUE, true.angle = FALSE, arrowfun=arrows,...)

Details

This function is useful because (u,v) may be in very different scales from the locations (x,y). So some careful scaling is needed to plot the arrows. The only tricky thing about this function is whether you want the true angles on the plot. For overlaying a vector field on top of contours that are the streamlines true.angle should be false. In this case you want u and v to be scaled in the same way as the x and y variables. If the scaling is not the same then the arrows will not look like tangent vectors to the streamlines. An application where the absolute angles are meaningful might be the hands of a clock showing different times zones on a world map. Here true.angle=T is appropriate, the clock hands should preserve the right angles.

See Also

arrows

Examples

#
# 20 random directions at 20 random points

x<- runif( 20)
y<- runif( 20)
u<- rnorm( 20)
v<- rnorm( 20)
plot( x,y)
arrow.plot( x,y,u,v) # a default that is unattractive 

plot( x,y, type="n")
arrow.plot( x,y,u,v, arrow.ex=.2, length=.1, col='green', lwd=2) 
# thicker lines in green, smaller heads and longer tails. Note length, col and lwd are
# options that the arrows function itself knows about.