STNET (CCP4: Supported Program)
- Generate stereographic net plot for use with polarrfn
The PLOT84 plot file on logical name PLOT is a stereographic net
intended to be printed on a transparency for overlaying on the
stereographic projection plots produced by
A stereographic net is used to measure the angle (shortest distance on
the surface of a sphere) between any pair of points on a stereographic
projection. (To measure the angular co-ordinates of a point, see
Each point on the projection corresponds to a point in polar angle
space, which represents an orientation of a rotated Patterson function.
Hence this gives the angle of rotation relating any pair of
For example, it could be used to identify the 90o rotations
relating any pair on monomers in a tetramer with non-crystallographic
point-group symmetry 222.
The angle must be measured along any one of the great circles
(projected lines of longitude connecting opposite poles - note that the
sphere is projected so that the poles are at the left- and right-hand
edges and the equator runs vertically!).
The other set of lines (projected lines of latitude parallel to the
equator) sub-divide the great circles and are calibrated in degrees to
allow the angle to be read off.
Note that the net used must have exactly the same diameter as the
plot, otherwise the results will not be accurate.
The plot file may be converted to PostScript or HPGL
for printing (resetting line width) with
pltdev -i file.plo -lnw .2
Print the PostScript file plot84.ps on your laser printer and
photocopy the stereographic net onto a transparent sheet.
The diameter shown in the title (currently 17.1cm) assumes you are
using size A4 paper. If not, the diameter should still come out
the same as your plots from Polarrfn. Just edit the title
in the call to PLTCTX and recompile. However if you want to make
a net with a specific diameter, you'll have to unravel the mysteries
of PLOT84 yourself - it probably involves fiddling with the window
parameters (see DATA statement for array W in the source).