function [xp,yp,tp]=get_day_xyt(zin)
% [xp,yp,tp]=get_day_xyt(zin) returns the daily 
% x,y positions of the drifters in zin at the 
% time tp. used for plotting day tick marks on
% tracks. assumes track ends at 00Z. dt=1 day.
% 4/7/01

% remove any initial nans
nnan=find(isnan(zin(:,1)));
zin(nnan,:)=[];
% find drifter id numbers
zin=[zin;1000*ones(1,8)];
nid=find(diff(zin(:,8))>0.01);
id=zin(nid,8);
Nid=length(id);
% form xp,yp arrays
xp=[];yp=[];tp=[];
for j=1:Nid
   m=find(zin(:,8)==id(j));
   mmax=max(m);mmin=min(m);
   k=[mmax:-4:mmin]';
   xp=[xp;zin(k,2)];
   yp=[yp;zin(k,3)];
   tp=[tp;zin(k,1)];
end
