Wednesday, March 6, 2013

Measurement artifacts in the Iditarod altitude plot

I just really don't like having altitude and temperature curves on the Iditarod speed plots, so when I start to look more closely at someone's "analytics" (I still cannot describe a speed/time plot as an "analysis" - sorry) the first thing I do is make them go away.  But today someone asked me a question about something he'd seen on a plot and while looking at his screen grab I noticed that someone's altitude was changing while the sled wasn't moving (i.e. when the speed was 0.0).  So, I decided it might be fun to take a look at someone who was parked a long time and see what happened.  Here's Martin Buser's 24 in Rohn:

As you can see, the altitude line is basically level but wiggles a bit (and I wish they'd get rid of the drop shadow on that line - it's pretty but it makes it a little more difficult to read the curve - more chartjunk!). How much does it actually vary?  Well, running my cursor back and forth and back and forth and back and forth and back and forth over the curve, the highest value appears to be 1489 feet on 3/5 at 11:10, and the lowest value appears to be 1309 feet on 3/5 at 1:40pm.  In other words, by the plot the sled lost 180 feet of altitude in 2 1/2 hours while sitting still.

So, what's really going on?  A couple of possibilities:

  • The tracker is using barometric pressure to measure altitude, and this is due to normal fluctuation or weather changes.  Not likely
  • The GPS in the tracker is triangulating altitude, along with lat/long.  This tends to be much less reliable than GPS lat/long readings and can fluctuate amore, and is the more likely explanation
Note that this represents an error range of about 12%.

[n.b. note that IonEarth will redraw the "analytics" while you're looking at it, taking it back out to the full scale even if you've zoomed in.  Thanks tons, guys - your stuff is a pleasure to work with.]

1 comment:

  1. In my experience with GPS receivers, that trianglate for altitude, 12% is about the best we get. As Melinda identified that is probably the explanation UNLESS something is haywire on the transmitter side of the GPS- which is doubtful.

    A bigger issue with me is I wish the blue line disappeared or went faint when the GPS is not moving for more than 10 minutes. The casual observer might get a false impression of the terrain as it is now presented.