marion mt. trail to SJ summit - how long is it really????

[FIGHT ON]

:roll:

[Perry]

I think Fight On's post makes a lot of sense. For awhile I've been wondering about the switchbacks/error factor and the 3D issue with steep trails. I suspect that a mathematics or engineering grad student could write a better algorithm using statistical methods and a high data sampling rate.

[AlanK]

I think Fight On's post makes a lot of sense. For awhile I've been wondering about the switchbacks/error factor and the 3D issue with steep trails. I suspect that a mathematics or engineering grad student could write a better algorithm using statistical methods and a high data sampling rate.

1. In several discussions on the San Gabriels boards, I gave a couple of examples of GPS measurements on trails with plenty of switchbacks that were accurate to 1% or so. That is good enough for hiking and at least as good as a wheel does on a rocky trail (I have good intentions to post some comments about wheel accuracy by people who have evaluated their problems on rocky terrain.).
2. Based on many repetitive posts on several boards, FO simply does not understand that it makes little sense to try to measure a hiking trail to better than a few percent. Considering the fractal nature of rocky trails, one cannot assign a unique length to them at all. FO implies that one can measure a trail without error and that is simply wrong.
3. The 3D problem is a non-issue. As I posted on the other board, a typical hiking trail has an average slope of 10% (528 feet gained/lost per mile) or less. For a 10% slope, the distance traveled along the sloping path is 0.5% further than the horizontal distance. Even on a steep trail (1000' per mile), the distance along the path is only 1.8% longer than the horizontal distance. This may be worth noting, but few hikes are measured that accurately.

[Perry]

Were those verified with a callibrated wheel? You can't just compare the GPS unit twice. And if you callibrate the wheel, you have to account for whether it's a 440yd track or 400m.

As far as the fractal nature, the length is relevant to an average walking stride. We have no interest in the hypothetical path of a caterpillar crawling along pebbles and sticks. Well, maybe Fight On is interested in arguing about that...

At 35% grade you are looking at a 6% error in distance calculations. This may seem pointless, but I do think Skyline is longer than 10 miles, finishing at the tram station. But I've never measured it. A few years ago I heard that somebody measured 12 miles from Ramon Road to the tram, but all these GPS-carrying people are saying 10 and 11. It sure doesn't feel like 10.

[Perry]

Speaking of fractals, this is my current avatar:


In 43 Megapixels:
http://www.palm-springs-photography.com ... pixels.png

[AlanK]

Were those verified with a callibrated wheel? You can't just compare the GPS unit twice. And if you callibrate the wheel, you have to account for whether it's a 440yd track or 400m.

I would never calibrate a wheel using a running track. Most people forget that the distance has to be measured a foot from the curb, etc. I used to measure road race courses (running). We calibrated the wheel using a straight path measured using a high quality steel measuring tape (and, yes, we included the temperature correction). We calibrated the wheel before and after the course measurement and used the difference in our estimate of the error, And, yes, contrary to some claims, there are errors in wheel measurements even on a straight, flat course.

In the simple GPS experiments I discussed, the mileposts on the Lower Sam Merrill trail represent calibrated wheel distances. My GPS agreed with those distances to 1%, and that is a path with many switchbacks.

The other experiment I showed compared tracks obtained by GPS with a trail on a topo map. Map measurements are usually short, but the measurements of the trail and GPS tracks were done the same way. It is the agreement between the GPS tracks and the trail measurement that is significant.

Those little experiments were only intended to show that GPS measurements are generally good enough for hiking, which has been my claim all along. GPS measurements can have problems and there are many discussions of the subject out there. But a measuring wheel is no alternative for hikers. They can be used to measure well-defined, fixed trails, but they are useless for most hiking situations because fewer than 1 in 1,000,000 hikers use a wheel when they deviate from the prescribed path, which is often.

As far as the fractal nature, the length is relevant to an average walking stride. We have no interest in the hypothetical path of a caterpillar crawling along pebbles and sticks. Well, maybe Fight On is interested in arguing about that...

My point in bringing up fractals is that there is no fixed length for a real path. One needs to set a scale and you are correct in focusing on a scale relevant to human strides. But it is important to realize that measurements with tape, wheel, pedometer, and GPS will all differ with each other and from trial to trial. There are errors in *any* measurement. One quickly realizes that getting a hiking trail mileage to a couple of percent is as good as, or better than, one needs.

At 35% grade you are looking at a 6% error in distance calculations. This may seem pointless,...

A 20% grade is very steep, as steep as any on Skyline. It is very hard to walk up a 35% grade. When one is traveling on territory with a grade steeper than about 25%, one needs to worry about what you called the 3D effect. But is is easy to take into account to a reasonable degree of approximation. Anyway, I have never heard of someone using a measuring wheel on a 35% grade!

...but I do think Skyline is longer than 10 miles, finishing at the tram station. But I've never measured it. A few years ago I heard that somebody measured 12 miles from Ramon Road to the tram, but all these GPS-carrying people are saying 10 and 11. It sure doesn't feel like 10.

I have measured around 11 from Ramon Dr. to the Ranger Station several times. I would need to dig out old notes to be more exact than that. It's a hard 11 for sure!

[FIGHT ON]

Speaking of fractals, this is my current avatar:

Perry! your avatar looks like one of those x ray slides that they show of persons head who just went through one of those MRI machines.
Like a section view. If you put a gps gadget right in the exact middle of it, and imagine that the yellow is the 18 foot x 100 foot bubble (but it would have to be like five times taller to be more to scale) all those blue dots would represent where the gps believed it was! Round and round the gps goes, where it thinks it is? EVEN IT DOESN'T KNOW!
The notion that these gadgets could some how accurately get a reliable measurement on a trail doesn't make sense.
Someone measured a distance of 0.96 mi between mile markers 1 & 2 on the way down from inspiration point. Sounds really accurate until you convert the error into feet. .04 (4%) of a mile is more than 211 feet! That's more than 70 YARDS OFF IN JUST ONE MILE!

I could take any measuring tape. nylon, steel what ever and be way closer than that. even if it was zero degrees on the up hike and 100 degrees on the return. 70 yards??! wow!

The guy who created this topic asked how long the "TRAIL" actually is!
Not how far it is 3 feet in the air, measured by some gadget swinging wildly from ones wrist in a huge bubble banging off the walls like a happy birthday balloon just got the air let out of it!
He wasn't talking about ants or insects either. Come on, we are talking about walking on the surface of a trail here.

1. If one is going to measure something one must start out with a reliable tool from the get go. 18x100 is crazy!
2. Then one has to use that tool properly! One can't possibly hope to answer this topic using a gadget moving wildly 3 feet above the surface of the trail. up and down forward and back all over the ding dang place! (Some models are worn on the wrist! )
3. If a gps is carried off trail it does not suddenly become more accurate! It still has that balloon all around it.
If one listens, one can hear the sounds of a pin ball machine!
#4. USE A WHEEL!

[KathyW]

Actually, a wheel will probably do a lousy job on a rocky trail.

There's a Nova episode on fractals:

http://www.pbs.org/wgbh/nova/fractals/program.html

[Perry]

Okay, I found the Lower Sam Merril Trail here:
http://maps.google.com/maps?f=q&hl=en&g ... UTZ0xTKQpQ

The switchbacks don't appear to be very tight, as it looks like a wide path about the width of a car. I'm curious how fast you were walking and what the sampling rate was. In other words, how far were you walking between each data point that the GPS recorded?

If you were to test a GPS unit on the 97 Switchbacks section on Mt. Whitney, hiking at a fast pace, and compare that to a measuring wheel that used a mountain bike tire, I think the GPS track would be significantly shorter. The tracking path would likely cut the switchbacks and simultaneously create a steeper virtual grade in excess of 35% (even though the trail is not that steep) which would create further inaccuracy. That said, I do think they are more useful than what Fight On believes.

[FIGHT ON]

Actually, a wheel will probably do a lousy job on a rocky trail.

Is more than 70 yards off per mile on a smooth trail accurate?