GeezerHiker

January 10, 2015

GPS – Just because you can doesn’t mean you should

Filed under: GPS, Hiking, Inaccuracy, North Cascades — Tags: , — geezerwriter @ 3:17 pm

After going back and forth since last June with the email support staff at Garmin Ltd I finally received a reply with some substance:

When the user uses anything but the one second track interval the track log is “missing” data.  We are putting a value for total distance in the GPX file in the track stats tag that gives BaseCamp the 1 second distance accumulation ( even if they are not setup for 1 second distance accumulation ).  So when the user removes the first point BaseCamp can no longer use our 1 second distance accumulation value and has to fall back on the old method of summing the delta between all of the points.  The problem is that all of the points are not there, because they are setup to record at a less than 1 second interval.  So when this happens they think they are just removing 1 point, while in reality they are removing possibly thousands of points along the entire track which is why the distance changes.

The substance is that they have finally acknowledged what I have been saying for the last six months and what I suggested in the last paragraph of my first posting on this issue back on June 10, 2014. That’s the good news; the bad news is that they clearly seem to think that substituting in  the “1 second distance accumulation” is a good idea! If it took six months to get them to even admit what they are doing, how long will it take to make them see that what they are doing is just plain wrong?

The Lily and Lizard Lake Trail, Skagit County, WA - 687 points provide a lot of detail.

The Lily and Lizard Lake Trail, Skagit County, WA – 687 points provide a lot of detail.

A Quick Review

After a lot of experimentation, I have my Garmin GPSmap 62stc set up to record a track log point roughly every 20 feet (6 meters). Since we usually average only one or two feet per second when hiking uphill, Garmin’s “1 second distance accumulation” would mean recording a point every foot or so – ten or twenty times as much data. For example, on the first half of last Thursday’s hike I recorded 687 points in roughly 4 miles, at an average distance of about 30 feet, as shown in the picture; the elapsed time was about 2:37 so the 1 second method would have totaled up around 9,400 intervals of about 2 feet each. The total track length (by the 1 second method) was listed as 4.7 miles. When I deleted a single point and forced a recalculation (“losing” almost 9000 points) the length changed to 4.2 miles.

Why my points were 30 feet apart rather than the 20 I asked for is something only Garmin could answer, but it is not a big deal: 687 points is more than enough to make a very good picture of the trail we were on. The track agrees very closely both with my previous tracks (made with different Garmin devices) and with published information and maps of this very popular local trail. We had good weather, we were moving at a good, steady rate with few stops and the device showed an accuracy between 10 and 20 feet (about as good as it gets) every time I checked (which was often).

The Big Question

Pay attention here, as I am pretty sure this is the point where Garmin is going to counterattack (assuming I can get them to respond before I die.)

Even on these winding mountain trails it’s not like we are juking rapidly from side to side as we go along – with the exception of the occasional 179° switchback, you are usually traveling more or less in a straight line. I’ve been paying attention to that this summer – most of the time you can see straight ahead along the trail for 20 or 50 or even 100 feet, then there is a subtle change of direction and you go straight for another stretch. A lot of the trails, including large parts of this one, follow old logging roads and railroads (which are not prone to rapid direction changes) and have only a handful of sharp turns. I think if you joined us on a hike you would agree that if you could accurately record a point every 20 or 30 feet or so you would get a pretty darn good track, and it should contribute almost nothing (except for a whole lot of extra data) to the measured length of the trail to accurately record a few thousand intermediate points along the way.

So this is the first core of the issue – it is all about accuracy. How do we explain that half-mile discrepancy between the “1 second” method and the “20 feet” method? When I have done similar comparisons in town on a nice straight city street with a full, clear view of the sky (and where you can get a good independent verification of the distance) there was no such difference. Garmin is on record with its explanation: when I edit the track I am “removing possibly thousands of points along the entire track which is why the distance changes.”

And I agree completely! Where we differ is that Garmin seems to think that more is necessarily better, that those thousands of additional points must be an improvement. That is a common fallacy – more chocolate is better; more habanero pepper maybe not so much.

I think I made a pretty good argument that those thousands of extra points cannot increase the accuracy by more than few feet in a mere handful of sharp turns, nothing like a half a mile. My argument is that the 1 second method was adding in an additional 9000 tiny errors – that would be a positional error averaging only 3.5 inches per reading, and that on a device that is only claiming accuracy of 10-20 feet.

My Assignment

So my assignment (which I very much do choose to accept!) is to try convince a bunch of engineers who work and probably live in a place with very few hills, very few clouds and very few trees that there are situations where a lot of their precious data is more like pepper than chocolate and is ruining the result rather than flavoring it.

And more poignantly, that this particular kind of pepper never improves the dish – most of the time is does no harm but it can never help.

I intended to dig into the gritty geometric and arithmetic details that support this argument but this is more than enough for one day.

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2 Comments »

  1. Well, it’s good for you to have a project, Al, that will continue to take many years of effort. Keeps you on your toes, even blackened ones! 🙂

    Comment by Jan — January 11, 2015 @ 5:35 am

  2. Just a note to tell you how much I love your Blog and to encourage you to continue posting. Thank you for all of your previous posts.

    Comment by Greg Vander Houwen — August 11, 2015 @ 8:21 am


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