Friday, December 30, 2011

Funky GPS tracking

A growing number of distance dogsled races have been using GPS tracking devices to allow fans to follow along from home in near-realtime.  I think this has probably been the main driver behind the incredible growth of mushing as a spectator sport, since you can't really be a spectator without something to spectate.

Nearly all races that provide online tracking are using inexpensive Spot trackers combined with a free online service called Trackleaders.  Spot trackers are GPS units with a satellite uplink to allow the sending of a small number of pre-programmed messages.  They were originally intended to be used as a way for people in remote backcountry who get into trouble to signal for help, but their use has grown tremendously to encompass a huge number of other uses.  I hit the "Send message" button on mine when I catch a fish on the Chena, as a way of mapping how many fish I'm catching in which locations.  The Spot also has a tracking option, to allow you to uplink location information every 10 minutes.  You can easily come up with similar stuff.

Anyway, the Gin Gin 200 is winding down, and as usual a lot of people found the tracking to be confusing, compounded by problems with data not being uploaded and a very, very unusual occurrence: some people appeared to be running the race backwards.

So here's what happened: a portion of the Gin Gin trail was a large clockwise loop, with a checkpoint at the far end of the loop.  The mushers were to run west down the Denali Highway to the Maclaren River, turn south on the river, turn north on the Susitna from its confluence with the Maclaren, get back on the Denali Highway and head east a short distance to the checkpoint at the Alpine Creek Lodge, do their mandatory layover, and continue east on the Denali Highway to the finish in Paxson.  Unfortunately high winds and deep snow had made trail conditions on the rivers extremely challenging and a few mushers chose to scratch but to continue to run west on the Denali Highway as a training run.  This meant that they were running in the "wrong" direction, with their trackers still sending out location data.

The other problem is that there were a lot of missing data - data not being uploaded had to do with the GPS antennas not having a clear view of the sky.  The race organizers said that they were experimenting with the placement of the Spots on the mushers and it looks like it didn't work out very well.  The result was that data were often hours old, or more.

Here's what the leaderboard at Trackleaders looked like a few hours after the last finisher arrived:

[Note that the times are being shown relative to the start of the race so, for example, Brent is shown leaving Maclaren Lodge six hours and 37 minutes after the start of the race rather than at 6:37.  I believe this format is useful for race officials but confusing for spectators.]

This table was generated from the GPS location data provided by the Spot devices.  There are several odd things here, the oddest of which shows Paul Gebhardt out of Maclaren Lodge, past the confluence, off the Su, and into Alpine Creek Lodge all at the same time.  You see similar problems with the data from people who scratched but ran to Alpine Creek.  I believe that while there are different proximate causes in the two cases, the root cause is the same: poor handling of problematic data by the software developer.  In Paul's case his GPS was basically not working at all until, I think, someone at Alpine Creek Lodge reset the device or otherwise fixed the problem.  In the case of someone running "backwards" (they don't show up in this screen grab), the situation probably looked similar in terms of the data - the dog team should have been through, wasn't, and so they just filled in the timestamp from the last Spot uplink.

There's always so much confusion about the tracking data, particularly when data are late or missing or someone's doing something wacky (just for fun, a screenshot from last year's Copper Basin.  The musher had left his Spot at a checkpoint and someone put it in a car and was driving it to the next checkpoint.  This led to some confusion among fans.  Me, I wondered what Johannes was feeding his dogs.)

I think in general it's safe to assume that if it looks weird it probably is, and if you're wondering what's going on good rules of thumb are:
  • Check the time of the last update
  • Trust the GPS data more than a summary of the GPS data (for example, if you can see where someone is on the map and the data are current, but a leaderboard claims they're somewhere else, trust the GPS data)
  • Sturgeon's Law says that 90% of everything in crap.  When it comes to software we're well north of 95%, so if something looks peculiar consider the possibility of a programming mishap.  In this case the programmer developing the leaderboard table did not make particularly good decisions about what to do when data are missing

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