function tripod_pca(az,ua,va,ub,vb)
%
% az is MAGNETIC azimuth from adv compass (assumes mag corrn is 19 deg)
% u and v are column vectors with x and y data
 
% Chris Sherwood, USGS
% Aug. 6, 2001

magcor = 19;
rot = az+90-magcor;
trot = az-magcor;

% calc and plot tripod info
sft = 1; % scale for tripod size
feet = [1.62,60;1.62,180;1.62,300];
feetcol = {'G';'B';'R'};
advloc = [.38,270-26.4;.38,90+26.4];
advsn = {'231';'244'};

[fx,fy] = xycoord(sft*feet(:,1),feet(:,2)+trot);
[ax,ay] = xycoord(sft*advloc(:,1),advloc(:,2)+trot);


for iadv = 1:2,
if(iadv == 2),ua=ub;,va=vb;end
  
% process first set of data from adv A == sn 231
[as,adir]=pcoord(ua,va);
as = as/100;
adir = adir+rot;
[u,v]=xycoord(as,adir);

mu = mean(u);
mv = mean(v);
C = cov(u,v);
[V,D] = eig(C);

x1 = [.5*sqrt(D(1,1))*V(1,1);-.5*sqrt(D(1,1))*V(1,1)];
y1 = [.5*sqrt(D(1,1))*V(2,1);-.5*sqrt(D(1,1))*V(2,1)];
x2 = [.5*sqrt(D(2,2))*V(1,2);-.5*sqrt(D(2,2))*V(1,2)];
y2 = [.5*sqrt(D(2,2))*V(2,2);-.5*sqrt(D(2,2))*V(2,2)];
[mspd mdir]=pcoord( mu, mv );
[ majr, majaz ] = pcoord( x1(1), y1(1) );
[ minr, minaz ] = pcoord( x2(1), y2(1) );
if(majr < minr),
  ltemp = majr; aztemp = majaz;
  majr = minr;    majaz = minaz;
  minr = ltemp; minaz = aztemp;
end
sd1 = 2*majr;
sd2 = 2*minr;

% plot data
step = 1;

h = plot(u(1:step:length(u))+ax(iadv),v(1:step:length(v))+ay(iadv),'.k','markersize',1);
set(h,'color',[.4 .4 .4])
hold on

% write pca data
ts = ['ADV ',sprintf('%s',advsn{iadv}),...
      '; Speed= ',sprintf('%5.2f',mspd),';  Direction= ',sprintf('%3.0f',mdir) ];
h = text(1,-.6-.6*(iadv-1),ts);
ts = ['Major axis: Mag= ',sprintf('%4.2f',majr*2),';  Az= ',sprintf('%3.0f',majaz) ];
h = text(1,-.8-.6*(iadv-1),ts);
ts = ['Minor axis: Mag= ',sprintf('%4.2f',minr*2),';  Az= ',sprintf('%3.0f',minaz) ];
h = text(1,-1-.6*(iadv-1),ts);

sf = 10;
% draw ellipse
theta=2*pi*(1:64)/64;
theta=[0 theta];
xx=majr*cos(theta);
yy=minr*sin(theta);
angle=-majaz*pi/180+pi/2;
xxx=sf*(xx*cos(angle)-yy*sin(angle));
yyy=sf*(xx*sin(angle)+yy*cos(angle));
h = plot(xxx+ax(iadv),yyy+ay(iadv),'-c','linewidth',2);

% draw major/minor axes
h = plot( sf*x1+ax(iadv),sf*y1+ay(iadv),'-c', sf*x2+ax(iadv),sf*y2+ay(iadv),'-c','linewidth',2);

% draw mean current
h = plot( sf*[0;mu]+ax(iadv),sf*[0;mv]+ay(iadv),'-b','linewidth',3);

end

% draw tripod
plot([fx;fx(1)],[fy;fy(1)],'--k')
hold on
for i=1:3,
    h = text(fx(i),fy(i),feetcol{i});
    set(h,'horizontalalignment','center');
end
plot(ax,ay,'ok');
for i=1:2,
    h = text(ax(i),ay(i)-.05,advsn{i});
    set(h,'horizontalalignment','center');
    set(h,'verticalalignment','top');
end

axis([-2 2 -2 2])
axis('square')