Another GPS question

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Another GPS question

Postby zippetydude » Thu Nov 07, 2013 3:30 pm

I'm wondering about how well the track-back function works on a current model GPS. I'm doing a lot of exploring xc nowadays and I often go up into areas where there are sheer rock faces that I have to work my way around as I ascend. I'd like to be able to find my exact path on the way back down, so it would need to be accurate to within just a few feet. If it's off by too much I can find myself cliffed in and potentially trapped. I'd like to avoid that. Any first hand experience with accuracy on track-back?

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Postby Perry » Thu Nov 07, 2013 7:52 pm

XC travel? Is this the same zip?
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Postby jfr » Thu Nov 07, 2013 9:04 pm

Sorry, zip. It's a nice idea but it won't work unless you have a military-grade gps unit, which can decrypt the higher quality gps signals. We common mortals have to make do with +/- 50 feet or so. Not quite good enough for perfect re-tracing of your route.

From the wikipedia article: "Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military."

You'd be better off making your own set of marks with flour.

Or maybe fluorescent orange spray paint? (Ducking and running...) :D
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Postby zippetydude » Thu Nov 07, 2013 11:59 pm

jfr wrote:
Or maybe fluorescent orange spray paint? (Ducking and running...) :D


Yeah but everyone got so excited after all my helpful dots on Skyline!

Hi Perry. Yes, my first love is really exploring, not running. Once I've done a trail a few times, I begin to burn out. I'm only about 50 into Skyline, same for San G and San B, and I find myself deserting the trail in new, random directions. I'm afraid I'll never get to the 200 to 400+ trips of some good friends. But I will end up all over the place, and I hope to see lots of things I've never seen before.

So, jfr, the public version is still kinda weak, huh? I guess I'll have to try out a combination of ducks and trackback and see if I survive. Wish me luck!

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Postby arocknoid » Fri Nov 08, 2013 12:15 am

white blazemarker creep paper cutouts of Hansel mitt Gretel?

Cheetos on an eight inch twig in a ziplock?

Pink duck tape?

(ducks n dodges ;)

Seriously, off piste exploring offers delightful discoveries as you well know, zip.
Another tool to consider having if needed for a bread-crumb-trace return pathway is a small cyalume marker, size of a two inch pencil stub, which you can hang or affix in a "can't miss being seen" location for any crucial junction if you may be returning in the dark. You can spot it in daylight, also. You can get a dozen for two bucks, and one or two in your kit will weigh less than an ounce, with a loooong shelf life and 8-12 hr runtime depending on color. These have other uses with outdoor activities, too.

(edit--if we meet on the trail sometime I'll tell you the hiking tie-in with the discovery and naming of luciferin and luciferase, and much more; for now, a salute to a departed dear mentor...RIP Bernie....)

kind regards, zip

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Postby Hikin_Jim » Fri Nov 08, 2013 2:04 pm

jfr wrote:From the wikipedia article: "Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military."
Is that still current? I know that when GPS first came out, they had a "futzing" routine that only military GPS receivers could de-crypt. But some clever civilians figured out some way to "average" (or something like that) the signals so that they could get a more accurate reading. The military then removed the "futzing" figuring that any sophisticated adversary could do what the civilians had done and that encryption was therefore pointless.

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Postby Myth » Fri Nov 08, 2013 2:26 pm

I have found my GPS ( Garmin eTrex 20 ) to be quite accurate, but not 100% accurate.

Depends on terrain, too. If you're in the bottom of a canyon or on a steep slope, the GPS may not have all the satellites it needs and accuracy may be degraded. So I wouldn't say it is accurate to within a few feet.

It's been accurate enough for me so far, and I have scrambled some tricky slopes and navigated terrain in JTNP's wonderland of rocks with it. Generally, when overlaying the track in Google Earth later on, I found it to be very accurate, showing which side of large boulders I passed by on and so on.

But I still wouldn't trust it to be pinpoint accurate in all important situations.
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Postby zippetydude » Fri Nov 08, 2013 3:12 pm

Wow, that's encouraging. I'm hoping HJ is right (leave it to the government to be "futzing" things up!) in that the accuracy is at full capacity nowadays. I'm still watching the other GPS thread to see if there are more recommendations about a cheap, easy to use GPS that's accurate enough to fulfill this specific function.

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Postby turtle » Fri Nov 08, 2013 3:49 pm

Hikin_Jim wrote:
jfr wrote:From the wikipedia article: "Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military."
Is that still current? I know that when GPS first came out, they had a "futzing" routine that only military GPS receivers could de-crypt. But some clever civilians figured out some way to "average" (or something like that) the signals so that they could get a more accurate reading. The military then removed the "futzing" figuring that any sophisticated adversary could do what the civilians had done and that encryption was therefore pointless.

Jim, you're thinking of Selective Availability. This "futzing" was the deliberate degradation of the satellite signals in a manner (the addition of a pseudorandom signal) that only the military could characterize (via knowledge of the seed of the pseudorandom sequence) and reverse.

Selective Availability is no longer active. And will likely not be active again for the reason you note. The "averaging" you refer to is actually the technique known as Differential GPS (DGPS).

As far as I know the only factors currently limiting the accuracy of civilian GPS receivers are the inherent limitations of the GPS system. But there are some good solutions to addressing these limitations. Those civilian users willing to pay for units employing these techniques (geologists, surveyors, precision farmers) can enjoy centimeter-level accuracies.

Zip, unless you're both really motivated and really well funded, these units are generally too heavy and expensive for outdoor use. But there are some indications that this may soon change.
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Postby climbant » Fri Nov 08, 2013 3:58 pm

On a sheer cliff I would say no. Even steep switchbacks aren't super accurate on the GPS but I think its close enough.

Jim, while the Government says they don't mess with the signals anymore, civilian GPS isn't as accurate as military grade. Whether you believe the government or not...
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