StCoordLine.m

function [xp,yp] = StCoordLine(trd,plg,sttype)
%StCoordLine computes the coordinates of a line 
%in an equal angle or equal area stereonet of unit radius
%
%   USE: [xp,yp] = StCoordLine(trd,plg,sttype)
%
%   trd = trend of line
%   plg = plunge of line
%   sttype = An integer indicating the type of stereonet. 0 for equal angle
%            and 1 for equal area
%   xp and yp = Coordinates of the line in the stereonet plot
%
%   NOTE: trend and plunge should be entered in radians
%
%   StCoordLine uses function ZeroTwoPi
%
%MATLAB script written by Nestor Cardozo for the book Structural 
%Geology Algorithms by Allmendinger, Cardozo, & Fisher, 2011. If you use
%this script, please cite this as "Cardozo in Allmendinger et al. (2011)"

% Take care of negative plunges
if plg < 0.0
    trd = ZeroTwoPi(trd+pi);
    plg = -plg;
end

% Some constants
piS4 = pi/4.0;
s2 = sqrt(2.0);
plgS2 = plg/2.0;

% Equal angle stereonet: From Equation 1.5 above
% Also see Pollard and Fletcher (2005), eq.2.72
if sttype == 0
    xp = tan(piS4 - plgS2)*sin(trd);
    yp = tan(piS4 - plgS2)*cos(trd);
% Equal area stereonet: From Equation 1.6 above
% Also see Pollard and Fletcher (2005), eq.2.90
elseif sttype == 1
    xp = s2*sin(piS4 - plgS2)*sin(trd);
    yp = s2*sin(piS4 - plgS2)*cos(trd);
end

end