##
GPLOT recipes: Polarisation vectors in a plot

*
The image shows E-vectors of the polarisation of NGC1265. The dashed contour
indicates the 7.5 mJy level of the total intensity of NGC1265 (Data: Dolf Sijbring)
*

### The GPLOT macro:

For a plot with polarisation vectors, you need a set with intensities (**inset**)
that represent the vector lengths, and a set with numbers in degrees or radians
(**polset**) that represent the vector angles (angles with respect to the north). The
default angle mode is radians, but this can be changed with command **polmode**.

inset polintclipped0.25 0 ! Set with intensities --> vector lengths
box 20 102 d 62 62 ! Box centered at 20 102, sizes 62x62
xsize 75 ! Plot sizes 75x75 mm
ysize 75
charheight 4.8
axdelta 20 HMSSEC ! Step for major ticks alonng X axis
axpos * 3 15 20 ! Write first label at 3h15m20s
axformat hmS ! Always write seconds
axis bp
axtitle R.A. (1950)
axpos * 41 42 0
axdelta 3 arcmin
axis lp
axtitle DEC. (1950)
axis tpa00 ! Top axis has no labels
axis rpa00
polmode R ! Angles are in radians
polset polang 0 ! Read the (sub)set with the vector angles

The polarisation vectors will be plotted with command **polplot**. This command
controls two options. The first two numbers are integers that set a grid mask. For
instance **polplot 2 3** will plot vectors on each second grid in the x direction
and on each third grid in the y direction. The default is 1 1, i.e. on *all* grid
positions a vector will be plotted. A third number in the **polplot** command
sets the conversion of an intensity to a vector length in grids. If you want to fit
the longest vector in just one grid, find *Imax*, the maximum intensity in the data
specified with **inset**, and set the scale to 1/*Imax*. If you omit this
number, a default will be calculated, so that the longest vector fits in one grid.
polplot 2 2 1/0.9 ! Plot the contours
inset 49cm 0 ! Total intensity map for the contour
lstyle 2 ! Select dashed line style
levels 7.5 ! Select one level
conto ! Plot the contour