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Thread: Watch the GPS or the Trail

  1. #61
    Senior Member Nessmuk's Avatar
    Join Date
    Dec 2004
    western 'daks
    Snow covered trails can be difficult to follow, but even in summer mistakes are made that can lead one astray on what should be easy to follow trails.

    I was instructing an annual wilderness guide training course when hiking on the way into an area for an overnight stay we came upon an intersecting trail coming in at an angle to the main trail in a "Y". I stopped the group to specifically point out a misshapen birch tree at the "Y" intersection. Note also that the main route is a well traveled canoe trail.

    Normally I follow the group at the end of the line and a designated student as group leader makes all the decisions (the students take turns being "leader of the day"). I only intervene in case of impending safety issues, but not for navigation errors. I let those errors play out with whatever consequence follows. The next day on the way out, sure enough the student guide by passed by that birch on the wrong side, heading down the wrong trail. Now this odd diverging trail ran offset but semi-parallel to the desired trail, but with a spruce swamp in between. So on what is supposed to be a canoe carry trail, we are walking through muddy spots with no previous tracks (such as ours from the day before), and there are low hanging trees that would make overhead canoe carrying impossible. No one seemed to notice these facts or to remember the details of the correct trail which had curves and small hills. I just let this go on for as far as they would take us. Finally after about a half mile one of the students thought something was wrong. A discussion, not quite an argument ensued with the leader and other students. I just shrugged my shoulders when they looked at me. Looking at the map, some wanted to cut across the swamp to reach the right trail and where we left our canoes. Ultimately, we backtracked to the missed intersection as was the correct decision. I had to remind them of that odd tree, and why I always turn around to observe the reverse view whenever I come across an intersection or other location of note. All ended up well, everyone passed the course that year, but not without a strong critique.

    I'm glad they made this mistake ( and several others during the week of training). When I was learning the craft, I made many mistakes, but learned good things from each one and resolved to never let that happen again. Luckily none were fatal or extremely serious, but had I continued and not recognized some of the worst mistakes early, they could have been. Learning by mistake, and analyzing how and why the mistake was made, IMO was the best way to learn. I never forgot what my Air Force instructor navigator once told me so many years ago. I can still hear him while standing over my shoulder saying: "Every navigator will make mistakes. The difference between a new navigator and an experienced one is how quickly the mistake is recognized and corrected." I have lived by that advice ever since in my air and land navigation endeavors.
    Last edited by Nessmuk; 02-22-2019 at 03:17 PM.
    "She's all my fancy painted her, she's lovely, she is light. She waltzes on the waves by day and rests with me at night." - Nessmuk, Forest and Stream, July 21, 1880 [of the Wood Drake Canoe built for him by Rushton]

  2. #62
    Senior Member Grey J's Avatar
    Join Date
    Jul 2013
    Chattanooga TN
    "I always turn around to observe the reverse view whenever I come across an intersection or other location of note." Nessmuk

    Possibly the best piece of advice contained in this entire thread.
    "I am a pilgrim and a stranger"

  3. #63
    Junior Member
    Join Date
    Jan 2018
    Quote Originally Posted by bikehikeskifish View Post

    There are many things that contribute to accuracy of GPS, only some of which relate to the device and some of which relate to the user. Even in what I consider the most controlled circumstances that I can provide, there is considerable variability.

    Use Case 1 - Stem Mounted, Garmin 520 Cycling GPS and my last 100 bicycle commutes to work, all following the same route. I zoomed in on a corner as that had the largest amount of visible deviation.

    Use Case 2 - Pocket Mounted Apple iPhone 6s and my last 100 times walking my dog in the neighborhood. Again I zoomed in on a corner.

    There aren't walls or cliffs or other obstructions in these corners. There are leafs and ice and snow and other seasonal changes.

    Use Case 3 - Backpack mounted Garmin 76CSx, Out-and-Back in a snowshoe track.

    While I don't feel very qualified to talk about most of everything else discussed in this thread, I know a thing or two about GPS. What you see above, especially the last image, are very typical of consumer-level GPS units (a lot of which use the same SIRFstar chip) - 1Hz update rate, tracks all over the place. Even on multiple outings your tracks will not overlap. Accuracy depends on where you are in the world, how many satellites are overhead, where they are in the sky, what the tree canopy looks like, what the cloud layer looks like, what's going on in the troposphere/ionosphere... so - accuracy of several meters (<7.8 with 95% probability).

    If - like already mentioned - you can get a signal from a differential station (DGPS), you can bring that down to 10-15cm.

    If you have several thousand $ to burn and you can get an internet signal in the mountains, you can get an RTK (real-time kinematics) correction that brings you down to 1cm. Yes, survey-grade 1 centimeter! And you can pay to get updates more often than 1Hz. Having most of my experience with jumpy consumer-level devices, it was kind of amazing and unbelievable to see how steady and precise RTK-DGPS can be.

  4. #64
    Senior Member DougPaul's Avatar
    Join Date
    Jan 2005
    Bedford, MA; Avatar: eggs anyone?
    Quote Originally Posted by Remix View Post
    I just did some checking...and no, unfortunately, we cannot submit our past GPX files to Opus and have them post processed to highly accurate tracks...OPUS only accepts data from survey grade receivers...
    A GPX file does not contain the information required to perform the correction. You need the pseudo-ranges. DGPS corrects errors in the pseudo-ranges before using them to compute the location, velocity, and time.

    There are also techniques (eg dual frequency) not used in typical hiking GPSes that can increase the accuracy.

    Last edited by DougPaul; 02-23-2019 at 11:34 PM.

  5. #65
    Senior Member Mike P.'s Avatar
    Join Date
    Sep 2003
    Colchester, CT
    On a prior page, their was some discussion on knowing bearings and names of points out in the field. Like others here, I am comfortable identifying the prominent features from bare summits that I care about. (Am I getting Big Attitash and Tremont right? Am I missing a few of the hills north Cannon or Lafayette, looking towards VT but not all the way to Mansfield, liek Hunger and the Worcester range, probably, however I don't really care either.) We all hike our own hike.

    I've brought a map out on a good day and oriented it with a compass, what I can't do is get teens (mostly scouts) to take the time and effort. If you can see the general area of your home, that might be nice. However, if you are a tourist from 150 -240 miles away....

    We were on Eisenhower a few years ago & most of the group saw all they cared to see, a wide open vista and where we planned to stop for lunch. They could Identify the Mt. Washington by the Red Roof, Monroe and Washington were easy enough and the near by notch beyond Pierce was Crawford Notch. None of them, except maybe my son will get to Tom, Field & Willey and highly unlikely they will get there in the future unless a SO is part of the UNH or Dartmouth Outing Club. I'll personally buy my son a GPS if one day he honestly says, I'd like to climb Mt. Dartmouth and The Captain nest weekend.
    Have fun & be safe
    Mike P.

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