STNET (CCP4: Supported Program)

NAME

stnet - Generate stereographic net plot for use with polarrfn

SYNOPSIS

stnet plot file.plo

DESCRIPTION

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 polarrfn. 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 STGRID.) 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 orientations. For example, it could be used to identify the 90^o 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:

   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.

BUGS

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).

AUTHOR