Algorithm

subroutine omdrifthist
  open th_file
  read x,y pairs 
  compute incremental tracking drift array
  compute  mean x,y 
  compute sigma x,y 
  compute number of times modulus(x,y) exceeds track_radius
  open postscript file 
  append exposure details 
  plot incremental drift size histogram 
  append tracking history stats (e.g means and sigmas) 
  close postscript file 
  end

end subroutine  omdrifthist