Sunday, March 4, 2012

The Iditarod trackers, and "trail miles"

IonEarth redesigned their tracking system between the last Iditarod and this one, and with the teams just having left Willow we're getting an opportunity to look at the thing and try and figure out how it works, what quirks might be confusing, etc.  It turns out that the trail mile representation might be really confusing to a lot of people.

The first thing to point out is that the red line on the map, which represents the trail, doesn't actually follow the trail.  It's pretty clearly the case that someone drew particular points and then lines were drawn between the points.  This is different from either drawing the trail in continuously freehand, or sticking a GPS on a snowmachine, sending it down the trail, and recording the track.  So, using this approach you get something like this:

Obviously the trail is going to stay on the river and not run up hills (in a perfectly straight line, no less), so it's no surprise to see dog teams deviating pretty far from the "trail."

Anyway, the first thing I wanted to look at was distance calculations (which is, of course, related to speed), so I took two recent GPS readings off Josh Cadzow's track: one at 4:20pm and one at 4:30 pm.  According to the tracker he was at mile 3.1 at 4:20 and mile 8.6 at 4:30, which means he would have been traveling at over 30mph.  He's fast, but ...  Eyeballing the locations on the map, alongside the scale provided on the left-hand side of the map, didn't look like he'd traveled anywhere near 5.5 miles.  So I did my own calculations on the distance between the two GPS readings (N 61°43.129', W 150°10.21' and N 61°42.903', W 150°12.748') and came up with 1.4 miles, which looks right and gives us a much more reasonable speed of 8.4mph.

The nerd instinct at this point is to roll up our sleeves and figure out what sort of arithmetic error would have led them to a result of 5.5 miles in 10 minutes, but a little more poking around found something odd:

and this:

In a nutshell, we've got Josh in two different places, each of which is labeled as being race mile 8.6.  The next obvious step was to calculate the distance between the 4:20 reading and the 4:40 reading, and rats!  2.52 miles -- not even close to 5.5.

Continuing to scroll through the data I found that there were 3 different readings which reported being at trail mile 3.1, so I tried the first of them (at 4:300pm), and came up with 5.1 miles.

So, obviously the calculations aren't reproducing the trails miles being reported and something odd was going on.  Chris and I started kicking this around and independently came to the same conclusion: one reasonable explanation is that rather than calculating mileage based on map locations, they're anchoring "trail miles" to those map points I mentioned up at the top.  That is to say, this:

When you're closer to one of these points to any other point, that's your trail mileage.

I don't know for a fact that this is what they're doing but it seems like a reasonable guess.  The distance between those two points certainly eyeballs out at 5.5 miles.  I hope IonEarth can tell us whether or not this is what they're doing.  I don't love it but dealing with a 1000-mile GPS track has got to be extremely challenging and while this would probably something of a kludge, it's not an unreasonable kludge.  But it definitely might be confusing to fans.


  1. Another thing I'm noticing is that the order of the mushers on the map does not always coincide with the position order in the list on the left hand side of the tracker. The mileage on the list seems to be rounded somewhat, so several mushers are listed as being at the same trail mile. On the map, their relative locations is clearer... if I'm reading that right.

  2. Right - the points on the map do show where they really are, but the "trail miles" means that they're within hollering distance (relatively!) of a known point, or at least that's what we think so far. So, a mess of people might be all shown at the same "trail mile" but can be as much as five miles apart. I think a lot of people will find this confusing.

  3. I would agree that IonEarth is using the very course route marked in red and when mushers are close to a point or node THAT is their race mile. That is a very CRUDE way of calculating things and I cannot see from what angle this could be helpful for the average race fan.

    The course route should certainly be more accurate - yep 1600 kms is a long way - but that would only be 6400 points if you had a point EVERY 250 meters... In today's computing world that is NOT a lot of data points.