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47897Re: GPS Routes

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  • TomJones
    May 1, 2010
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      I have to respectfully disagree with you here, my dear Cooprec, on several points.

      --- In Zion_National_Park_Hiking@yahoogroups.com, "mcooprec" <coopermb@...> wrote:
      >
      > For the benefit of people just beginning to learn GPS: elevation readouts on GPS devices are virtually worthless. They are inaccurate on account of physics: it's really difficult to gauge how high something is when viewing it from above, and that is essentially what the GPS satellites do when getting a fix on your position. This is one reason why altimeters are such a valuable navigation tool.

      TOM: GPS elevations are roughly twice as inaccurate as your lat-lon locations. Which makes them not so useful for contour-navigation, which is an excellent navigation trick in some conditions. These conditions rarely occur in Utah.

      The accuracy error comes because there are rarely satellites overhead. The satellites are spread out in the sky, and if you do the math, you can see that the error locus is much large for altitude. If you had satellites close to and below the horizon, the error locus could be much better, but we generally cannot get a good signal from those satellites, and they are farther away which increases their error locus.

      Perhaps an easier example to visualize of the same problem is to imagine yourself in a slot canyon, not too narrow, with a good view of a swath of the sky, and a good signal from 6 satellites - but the satellites are all in a row. For the example, our canyon runs north-south, so all our satellites are in a north-south row. Take waypoints in this situation and you will find that your position north-south is well-defined, but your position east-west is poorly defined. If your north-south accuracy is 10 feet (say), your east-west location will be off by 100 or 200 feet. Because all the available east-west information comes from satellites in a n-s row, each satellite has the same kind of e-w error and the information from the next satellite does not correct the e-w error in the information you already have. Adding more satellites does not make your e-w position better, as it does with your n-s position.

      The altitude problem is essentially a 3-D version of the 2-D slot canyon problem.

      But notice that since you already know you are IN the slot canyon, all you are really interested in is WHERE in the slot canyon you are, and an accurate N-S position does just that. Ignore that the GPS is giving you an E-W coordinate that says you are 200 feet to the side of the canyon.
      >
      > Altimeters save time and energy when traversing terrain (versus climbing or descending), as they allow you to avoid needless up and down travel when no obstacles are in your way. For example, when traversing around the head of a basin to get to an exit point you've marked on the map or using GPS, an altimeter makes it easy. And finding that hidden lake in deep forest (I'm from Oregon :-) )? An altimeter -- in conjunction with a map bearing before leaving the trail -- is sometimes indispensable.

      In places, Altimeters are a great navigation too. however, they have their own set of problems, especially if the weather is changing, because, really, they are barometers, made to read as altimeters.

      But really, just geek-naviagator talk. Good on ya!

      Tom
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