title: extract mean track from several time series of XY data
body: <p>I have 11,719 XY coordinate pairs obtained from 6 different instruments that track ocean currents (they measure UTC time, longitude, latitude, temperature). These instruments drifted along, so there are no repeat measures at the same location. I need to extract a "mean track" from that XY data. Here is the dataset: <a href="http://dropbox.com/s/dg0psl3hnmilg44/summerdrifters.csv" rel="nofollow">http://dropbox.com/s/dg0psl3hnmilg44/summerdrifters.csv</a></p>
  
  <p>I was thinking of a regression line but the mean track i want to get is obviously not linear. I tried using different fitting functions, such as smoothing splines through <code>cftool(lon,lat)</code>, but it is not the most convenient way as I need to fraction the data into subsets, then somehow merge the different functions.</p>