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

[some guy]

I think that "fight on" may be confusing accuracy with precision here. In my experience, where a gps says that it has a 18 foot error doesn't mean that it's bouncing around like a pinball 36 feet every second. It's generally pretty precise, and while standing still might jump around by a couple of feet every second.

The reported accuracy is 18 feet, meaning in its best estimation, the gps is giving your coordinates within 18 feet of your true location. It's precision seems to be much greater than that though, and the error from the true location seems to be consistent, for example consistently 7 feet to the southwest of your true location. Come back the next day with a different satellite configuration in the sky and it might be consistently 10 feet to the northwest. But for the length of a trail, it isn't important if the gps measures it 12 feet to the left of it's actual location.

Also, for what it's worth, there are high end gps devices with sub-meter accuracy real-time, and can be improved to centimeter accuracy with differential post-processing. But these require large backpack antennas in mountainous areas and under tree canopy. And honestly, I can't imagine why anyone would care about such precision for backcountry trails like this.

[FIGHT ON]

I think that "fight on" may be confusing accuracy with precision here. In my experience, where a gps says that it has a 18 foot error doesn't mean that it's bouncing around like a pinball 36 feet every second. It's generally pretty precise, and while standing still might jump around by a couple of feet every second.

The reported accuracy is 18 feet, meaning in its best estimation, the gps is giving your coordinates within 18 feet of your true location. It's precision seems to be much greater than that though, and the error from the true location seems to be consistent, for example consistently 7 feet to the southwest of your true location. Come back the next day with a different satellite configuration in the sky and it might be consistently 10 feet to the northwest. But for the length of a trail, it isn't important if the gps measures it 12 feet to the left of it's actual location.

Also, for what it's worth, there are high end gps devices with sub-meter accuracy real-time, and can be improved to centimeter accuracy with differential post-processing. But these require large backpack antennas in mountainous areas and under tree canopy. And honestly, I can't imagine why anyone would care about such precision for backcountry trails like this.

I think the guys at garmin may have misstated the stats of the forerunner 305.
They told me it was 18 feet. Looks like it's BIGGER!

Specifications
Type: Handheld

Acquisition Time Cold: 45 Seconds

Acquisition Time Warm: 38 Seconds

Update Rate: 1/second, continuous

GPS Accuracy Position: <10 meters, typical

GPS Accuracy Velocity: 0.05 meter/sec RMS

Battery Type: Rechargeable Lithium Ion Battery

Battery Life: 10 Hours

Antenna: Built-in patch

TEN METERS?! WOW!:shock:

[AlanK]

I have addressed FIGHT ON's issues and misconceptions on other threads. He ignores any comment he does not like and just repeats his beliefs over and over. I have given up trying to have a discussion with him. He does indeed confuse precision with accuracy, but his biggest mistake is in assuming that there are error-free measurement techniques. There was a decent GPS discussion on the San Gabriels board last week and a lot of the issues were covered.

Perry -- There has been some good work with GPS on the 97 switchbacks of Whitney. I will try to locate some of the old discussions on the WPSMB when I have time.

Perry -- Regarding the Lower Sam Merrill Trail hike, I was walking at 3-4 mph (different up and down) and the GPS points were 6 seconds apart (26 - 35 feet). The integrated distance for the slow (up) and fast (down) legs were well within 1% of each other and both agreed with the mileage posts to 1% or better. The trail is not much wider than, say, the Mt. Whitney trail but the Pasadena trail switchbacks are less dense than the 97 switchback segment.

By the way...!!! When one uses a measuring wheel (which I have done many times and FIGHT ON has yet to actually try), one travels slowly. Traveling at such speeds with a GPS reduces error due to rounding switchbacks, etc. If we are going to compare GPS and wheel, we should assume slow motion for both. Of course, having to go slowly is one reason why people don't hike with wheels. The other big reason is that they are cumbersome and take all the fun out of hiking.

I am thinking about doing some better work on quantifying the accuracy of GPS distance measurements on trails under various conditions. Before embarking on anything, I want to find out more about what is out there -- what has been done in the past. More on that later.

[FIGHT ON]

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

Staying on topic here.
There is no way anyone can answer this question with something swinging all over the place. It just simply can't be done. PERIOD! Is this the twilight zone?
THE GUY WANT'S TO KNOW HOW LONG THE TRAIL IS!!!

HOW LONG IS "IT" REALLY. "IT" BEING THE TRAIL.
The thing you WALK ON!

1. The question does not ask for an approximate guess.
2. Not an accumulation of straight line measurements between points 20 feet apart, in the air.
3. Especially swinging all over the place on your wrist!
4. The only thing that can make this gps gadget more accurate is to lower the bar on what is acceptable. Homey don't buy that. nope!
Trails ain't straight lines. They ain't one gradual incline that can be averaged out. They go up, and down, and this way, and that way, and zig and zag, and round the corner and gps does not pick this up.
When gps is accurate, then you would have to drag it on the trail to find out how long it really is.
A wheel does all this stuff already. All of it.

[AlanK]

I have addressed FIGHT ON's issues and misconceptions on other threads. He ignores any comment he does not like and just repeats his beliefs over and over. I have given up trying to have a discussion with him. He does indeed confuse precision with accuracy, but his biggest mistake is in assuming that there are error-free measurement techniques.

Actually, a bigger problem is simply not comprehending relative accuracy. Measuring a hiking trail to 1% accuracy is overkill for almost any purpose and it is not trivial to do. For example, if you have three people measure a trail with measuring wheels, they will get three different results that differ by ~1%. Now, 1% is about 50 feet. That sounds like a lot, which is true if you are measuring a backyard or even a carefully laid-out 5K road racing course. It also sounds like a lot when someone mindlessly repeats it over and over. But, for the length of a trail, 50 feet out of a mile is pretty meaningless. So is claiming that one has an easy way to do better.

A GPS makes position determinations at discrete points. Each point has some uncertainty -- all measurement have uncertainty. Talking about a circle around each point of some radius and saying that the measurement can fall anywhere inside ignores the fact that there is a Gaussian distribution that is peaked at the correct location. It is also wrong to ignore the fact that errors along the path of travel average to zero when one counts many data points. It distorts things in the wrong direction to claim that the error for each point is larger along the path length, as a certain someone repeatedly does when he takes the total distance inaccuracy over a mile path and assumes that it applies to every data point.

If you really wanted an accurate measurement of a trail on a human (as opposed to ant) scale, the best approach would probably be classic surveying techniques.

[FIGHT ON]

Other than telling time, I see no difference from a kid going to the dime store, plopping down a ten cents, buying a box of candy coated popcorn peanuts and a toy, and pulling out a two bit ring from a cracker back jox, putting it on and walking along the yellow brick road while holding it in the air announcing that it can beam me up scotty to mars and back and at the same time be in london just in time to see the queen! dream on!


foreunner 305

Specifications
Type: Handheld

Acquisition Time Cold: 45 Seconds

Acquisition Time Warm: 38 Seconds

Update Rate: 1/second, continuous

GPS Accuracy Position: <10 meters, typical

GPS Accuracy Velocity: 0.05 meter/sec RMS

Battery Type: Rechargeable Lithium Ion Battery

Battery Life: 10 Hours

Antenna: Built-in patch

[tomcat_rc]

another apples and oranges discussion

[AlanK]

another apples and oranges discussion

True enough, and time to end it.

I do think I have some insight into the complete lack of actual communication. Try talking to a guy who thinks that Cracker Jack costs a dime! It did once... before GPS was conceived. Now it costs about a buck.

[FIGHT ON]

another apples and oranges discussion

Not even dude! The wheel is on the ground, in constant contact with the surface to be measured. It can't be measured with some beacon somewhere off in the wild blue yonder! Aimlessly veering from side to side having no idea where it actually is in the first place! give me a break! If you buy into this gpsbs this then I got some lake side property right off the coast of Mt. Baldy for sale! I'll even throw in a few gps units for free! just fifty dollars down, fifty dollars a week for fifty years! and the notion of being within in 1% if it reads .01 difference then it's more than .01 off. it is more than 105 feet off!
I feel like we are all standing at some road side shack listening to some snake oil salesman trying to hoodwink innocent passers by. don't get sucked in dude! Use your head!
I AINT BUYIN IT! USE A WHEEL!!!!!

Imfightonandiapprovethismessage!

[KathyW]

True enough, and time to end it.

Well said.