Where’d the mile go?

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Raymond

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My gut is telling me no... but my gut is also very
I climbed North Twin on Saturday afternoon.

When I crossed the Little River (at the ‘‘third crossing’’ — I had taken the bypass path past the first two crossings), I checked my GPS. It read 1.99 mile. I knew it was supposed to be 1.8 mile at that point, but 1.99 was close enough, especially as I had had to park on the other side of the bridge.

As I grinded my way up the steep section, I checked the GPS and saw, to my dismay, a reading of about 2.6 miles. It sure felt like it had taken a long time for only two-thirds of a mile. When I recognized that I was nearing the summit plateau (a few raindrops had fallen, too), I checked again and saw that the mileage was 3.05 miles.

Yikes, I thought, it’s 4.3 miles to the summit, and I’m still more than a mile away? It didn’t seem possible.

I reached the cairn marking the viewpoint trail, went to the viewpoint cairn, checked out the two short herd paths to rocks in the krummholz which I suppose were made by hikers thinking they’d located the true summit, then returned to the rock back in the trees which is about midway between the two cairns (which to my eye appears to be the actual high point), out of the wind, and checked the GPS’s trip odometer: 3.51 miles.

But the signs and White Mountain Guide both state that the distance should be 4.3 miles.

I went back down, taking the actual trail this time across the river thrice, and checked the GPS when I reached my car: 7.69 miles. So that made it 4.18 miles for the descent, too big a difference, I think, to be explained by merely taking the bypass herd path during the ascent — and my distance for that route, remember, came out slightly longer than the book distance, anyway.

And then the saved track for the entire hike came out as 8.66 miles! — almost exactly twice 4.3.

So where did the mile go during the ascent? Was I just moving so slowly that the GPS didn’t realize I was moving?

The track looks normal on the Garmin National Parks East map in the computer; that is, the GPS didn’t turn itself off then back on again anywhere along the way (that’s happened before, for some reason — I get two active logs for one continuous walk). The moving time was about three-and-a-half hours, stopped time about one-and-a-half hours, so it definitely knew I wasn’t moving much at times, but I’ve never noticed distance vanishing like that before.

And an unrelated North Twin question: Were the trees on the summit plateau cut back from the trail since 2007? It seemed much tighter through the spruce last time.

Thanks.
 
Possibilities:
1. You inadvertently reset the odometer. (Buttons can be pressed by objects in your pack etc.)
2. You lost lock during the way in and the odometer didn't record that section. If this happened, there should be one or more unusually long gaps in the track. The software interpreting the track may simply draw a straight line through these gaps, thus giving a larger distance than the odometer.
3. Bug in the GPS software.

There may be other evidence in the recorded track, such as the track segments or times. (Some software silently combines track segments so this may depend upon the software used to read the tracks from the GPS. I have seen this happen in some versions of GPSBabel, but don't know what MapSource does.)

Doug
 
I'm not an expert on the subject but have had this discussion before. The odometer on the GPS sometimes does not measure steep well. Here's an example. You went from A to C but the GPS measures A to B.
images%2520%25282%2529.jpg


The track log seems to be more accurate on mileage. This has been my experience. But it's not a rule. Sometimes even when hiking steep, both the odometer and track log are very close. I guess it's not an exact science.
 
I think I found it doing Downes Brook last week. GPS had it as 7 miles on the way up (we did some meandering, but not 1.8 miles worth). I had it in my pocket, so I suspect that affects it. Our descent was more accurate though.
 
I'm not an expert on the subject but have had this discussion before. The odometer on the GPS sometimes does not measure steep well. Here's an example. You went from A to C but the GPS measures A to B.
images%2520%25282%2529.jpg


The track log seems to be more accurate on mileage. This has been my experience. But it's not a rule. Sometimes even when hiking steep, both the odometer and track log are very close. I guess it's not an exact science.

If you hike 4 miles and gain 3000', that's

5280*4= 21120' with 3000' of gain so the triangle is very narrow.

Using some old guy's theorem,

C**2 = 21120**2 + 3000**2

We get 21332' for C, which is 4.04 miles, so the .04 is negligible here.
 
Regarding Tom’s comment at the end here, that’s what I thought. I remember a couple of years ago DougPaul pointed out that the GPS’s straight-across-through-the-air distance from North Kinsman to Lafayette wouldn’t differ much from the actual walking distance because of the landscape’s ups and downs being relatively insignificant, despite appearances to the contrary. So I don’t think it was the steepness of the trail, unless it was how the steepness was affecting my hiking speed.

The track looked fine, with regularly spaced dots, nothing untoward.

I climbed Mount Monadnock on Wednesday, but there was so much up and down and back and forth on various trails because I was trying to red-line some that I’ve missed over the years that I don’t know if the total distance was screwy or not, yet.

Thanks for the ideas.
 
The track looked fine, with regularly spaced dots, nothing untoward.
If you care enough, you can compute sub-distances between points on the track log which will tell you if there is a bad point or if the program computing the overall result is screwy
 
Grey J: Ha! I don’t drink, though. (No alcohol at all, and not enough water either, for that matter.)

Roy, that’s a good idea. I’ll just have to wade through all those points sometime, which is an even bigger pain because I never got the GPS to work with the new computer, so I always have to haul out the old computer and set it up every time I want to dump the GPS tracks onto the maps.
 
If you hike 4 miles and gain 3000', that's

5280*4= 21120' with 3000' of gain so the triangle is very narrow.

Using some old guy's theorem,

C**2 = 21120**2 + 3000**2

We get 21332' for C, which is 4.04 miles, so the .04 is negligible here.

This assumes a constant grade, so the .04 is a minimum. If there is a substantial curve then the distance will start to approach A+B. If there is substantial up and down (think PUDs on the GRT), then it could even exceed that. This is meaningless though, because the North Twin trail is a beautiful constant grade that doesn't have enough of a curve to make you miss a full mile. I suspect you lost signal for a while.
 
I climbed North Twin on Saturday afternoon.


And an unrelated North Twin question: Were the trees on the summit plateau cut back from the trail since 2007? It seemed much tighter through the spruce last time.

Thanks.

Yes, The trees in the summit area have been cut back since 2007.
 
The 3D distance (including vertical) is generally only trivially longer than the 2D (horizontal) distance for the routes that we generally travel.

This is one of those recurring questions...


BTW, that is the Pythagorean theorem. http://en.wikipedia.org/wiki/Pythagorean_theorem

Doug
Ayup, just trying to (humorously?) imply that the formula is well known and goes back thousands of years.
 
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