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Re: GPS Routes

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  • jamesclark9
    Michael, I will look into the book. For only $12, it seems to be a wise investment. Those are a lot of skills I need to work on. I don t think I m going to
    Message 1 of 22 , May 1, 2010
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      Michael,
      I will look into the book. For only $12, it seems to be a wise investment. Those are a lot of skills I need to work on. I don't think I'm going to do any serious technical routes so I hope not to get turned around, but I'm sure no one plans on getting lost.

      I had a question. I understand where following a GPS route from someone else or using a GPS unit that you don't know how to use can get you into trouble. Is a GPS sufficient to use to backtrack the route that your GPS tracked on the way in? I know that doesn't replace having other survival schools that you menitioned. All these accidents in the park has me scared now.

      Thank you for the advice




      --- In Zion_National_Park_Hiking@yahoogroups.com, "mcooprec" <coopermb@...> wrote:
      >
      >
      >
      >
      >
      >
      >
      >
      >
      > James, I highly recommend you buy and read "GPS Made Easy" by Lawrence Letham. That book -- and tons of backcountry experience -- taught me how to use GPS safely.
      >
      > Joe is right: depending too heavily or solely on GPS can lead to disaster in places like Zion. You should also be proficient with map and compass (knowing how to read topos and do both field and map bearings, taking declination into account). I also recommend carrying an altimeter. Using all four tools (GPS, map, compass and altimeter) will keep you out of trouble.
      >
      > There was a story on this forum about three guys who accidentally ended up in Heaps Canyon when their GPS was off by about 1/10 mile. I suspect what might have happened is they had their reference set to the wrong datum, which can lead to coordinates being off by several hundred feet. Not having a full understanding of how GPS works and simply turning the thing on and following "highway" views to published coordinates can lead to a simple (and potentially fatal) mistake like that.
      >
      > Best wishes,
      > Michael
      >
      > --- In Zion_National_Park_Hiking@yahoogroups.com, "jamesclark9" <jamesclark9@> wrote:
      > >
      > > Hello All,
      > > I just bought a handheld GPS to use during my Zion trip. Now I just need to figure out how to use it. I want to use it to track the places I hiked and to ensure I don't get lost. I was wondering if there were any suggestions about where to download Zion hiking routes?
      > >
      > > Thank You,
      > > James
      > >
      >
    • jamesclark9
      So I have been researching a bit on Amazon. I noticed National Geo has a hard copy topo map of Zion. The price seems to vary a bit when I look online. In
      Message 2 of 22 , May 1, 2010
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        So I have been researching a bit on Amazon. I noticed National Geo has a hard copy topo map of Zion. The price seems to vary a bit when I look online. In addition National Geo has TOPO! National Geographic USGS Topographic Maps (Utah) and TOPO! National Parks electronic software that you can print maps from. Neither of these seem to have the extensive reviews that most Amazon.com products have. Does anyone have experience with these items?

        Thank You

        --- In Zion_National_Park_Hiking@yahoogroups.com, "jamesclark9" <jamesclark9@...> wrote:
        >
        > Joe,
        > I have a Delorme 30 GPS unit. I'm going to play around with it on several hikes in my local area to see how it works. It may already have have the trails on the included discs. If not the Delorme website has a membership where I can download alot of maps.
        >
        > What is the best way to purchase topo maps? I browsed a bit on the USGS website, but I figure they can be purchased other places.
        >
        > Thanks for all the advice
        >
        > --- In Zion_National_Park_Hiking@yahoogroups.com, "Joe Braun" <joe@> wrote:
        > >
        > > Hi James - Not sure which unit you have, but I bought the electronic maps for my Garmin and it displays all of Zion's official trails right in the display without adding anything else. But call me old school, I wouldn't rely solely on a GPS for navigation. Many people have become disoriented and have entered the wrong canyon by navigating by GPS without referencing a map or compass. Especially for backcountry routes, it's always good to carry a topo map to help you analyze where you are in relation to various landmarks.
        > >
        > > I offer a lot of maps on my site (using the UTM grid to match the NG map and the USGS topos), but I won't offer any electronic version for that reason. HTH, JOe
        > >
        > >
        > > --- In Zion_National_Park_Hiking@yahoogroups.com, "jamesclark9" <jamesclark9@> wrote:
        > > >
        > > > Hello All,
        > > > I just bought a handheld GPS to use during my Zion trip. Now I just need to figure out how to use it. I want to use it to track the places I hiked and to ensure I don't get lost. I was wondering if there were any suggestions about where to download Zion hiking routes?
        > > >
        > > > Thank You,
        > > > James
        > > >
        > >
        >
      • Ray Fink
        I have the National Geographic Trails Illustrated map of Zion NP, as well as their maps for many other parks. I think they are generally excellent, though of
        Message 3 of 22 , May 1, 2010
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          I have the National Geographic Trails Illustrated map of Zion NP, as
          well as their maps for many other parks. I think they are generally
          excellent, though of course they don't give you much detail as a 1:24K
          USGS topo.

          The TOPO! National Parks software works pretty well for printing custom
          maps, I use it as well as several other topographic software packages
          for trip planning and custom maps. The USGS maps are still the gold
          standard.

          Reid Preidhorsky on the Grand Canyon hikers list has done some
          comparisons of printed maps from various sources. Check those out online
          at http://reidster.net/trips/maps/

          -- Ray


          jamesclark9 wrote:
          > So I have been researching a bit on Amazon. I noticed National Geo has a hard copy topo map of Zion. The price seems to vary a bit when I look online. In addition National Geo has TOPO! National Geographic USGS Topographic Maps (Utah) and TOPO! National Parks electronic software that you can print maps from. Neither of these seem to have the extensive reviews that most Amazon.com products have. Does anyone have experience with these items?
          >
          > Thank You
        • mcooprec
          Hi, James: Using the backtrack function works well, with several caveats. First of all, you must turn on the track log function of your GPS at the start of
          Message 4 of 22 , May 1, 2010
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            Hi, James:

            Using the backtrack function works well, with several caveats. First of all, you must turn on the "track log" function of your GPS at the start of your route. Clear all existing points in the log at the start of your route. You will probably want to turn your GPS off periodically to save batteries if your route is fairly long. Conversely, in order for your GPS track log to be of practical use, you must turn the GPS unit back on often enough that it enters a sufficient number of positions into your track log (when you power up) to guide your return. So, at every possible juncture where you could make a wrong turn, make sure your GPS is turned on and your track log is on and recording your current position. If you don't record enough positions enroute, you'll need to remember how you got to recorded points 1>2, 2>3, 3>4 and so on. On your return route, simply invert the route and follow it backwards.

            All that said, it's very important that you constantly turn around and make mental notes of the way you came, so you'll have it memorized for your return trip (the route will look different to you on the return than it did going out). GPS units can fall and break or run out of batteries, in which case you must rely on your other navigation skills (including making mental notes of your route). Some older GPS units don't perform well in deep canyons, forest or up against very tall mountains; i.e., where there is not a very good "sky view."

            So, yes, a GPS track log works well. But relying solely on GPS, or any other device prone to failure or unexpectedly poor performance, can get you into trouble. This is the reason I like to use several navigation tools and techniques -- when they are _all_ in agreement as to my location, then I know I've got the route dialed in.

            Best wishes,
            Michael

            --- In Zion_National_Park_Hiking@yahoogroups.com, "jamesclark9" <jamesclark9@...> wrote:
            >
            > Michael,
            > I will look into the book. For only $12, it seems to be a wise investment. Those are a lot of skills I need to work on. I don't think I'm going to do any serious technical routes so I hope not to get turned around, but I'm sure no one plans on getting lost.
            >
            > I had a question. I understand where following a GPS route from someone else or using a GPS unit that you don't know how to use can get you into trouble. Is a GPS sufficient to use to backtrack the route that your GPS tracked on the way in? I know that doesn't replace having other survival schools that you menitioned. All these accidents in the park has me scared now.
            >
            > Thank you for the advice
            >
            >
            >
            >
            > --- In Zion_National_Park_Hiking@yahoogroups.com, "mcooprec" <coopermb@> wrote:
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            > > James, I highly recommend you buy and read "GPS Made Easy" by Lawrence Letham. That book -- and tons of backcountry experience -- taught me how to use GPS safely.
            > >
            > > Joe is right: depending too heavily or solely on GPS can lead to disaster in places like Zion. You should also be proficient with map and compass (knowing how to read topos and do both field and map bearings, taking declination into account). I also recommend carrying an altimeter. Using all four tools (GPS, map, compass and altimeter) will keep you out of trouble.
            > >
            > > There was a story on this forum about three guys who accidentally ended up in Heaps Canyon when their GPS was off by about 1/10 mile. I suspect what might have happened is they had their reference set to the wrong datum, which can lead to coordinates being off by several hundred feet. Not having a full understanding of how GPS works and simply turning the thing on and following "highway" views to published coordinates can lead to a simple (and potentially fatal) mistake like that.
            > >
            > > Best wishes,
            > > Michael
            > >
            > > --- In Zion_National_Park_Hiking@yahoogroups.com, "jamesclark9" <jamesclark9@> wrote:
            > > >
            > > > Hello All,
            > > > I just bought a handheld GPS to use during my Zion trip. Now I just need to figure out how to use it. I want to use it to track the places I hiked and to ensure I don't get lost. I was wondering if there were any suggestions about where to download Zion hiking routes?
            > > >
            > > > Thank You,
            > > > James
            > > >
            > >
            >
          • mcooprec
            James, I also have the National Geographic Trails Illustrated map for Zion National Park. If you are venturing off-trail, this map alone may not suffice for a
            Message 5 of 22 , May 1, 2010
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              James, I also have the National Geographic Trails Illustrated map for Zion National Park. If you are venturing off-trail, this map alone may not suffice for a safe trip. The contour lines are 100 feet apart, which omits a lot of detail. The USGS maps, on the other hand, provide either 40- or 80-foot contours, depending on the specific area you are viewing. TOPO! Utah provides digitally scanned versions of the USGS maps and is an excellent product. But what makes the USGS maps so valuable is their large format: you can really see a lot of terrain at full resolution (i.e., smallest contour interval) on one large sheet of paper, which gives you a perspective of how all the landscape fits together vis-a-vis your route.

              TOPO! products do have two huge advantages (besides lower cost if you're using many maps) over USGS maps, however: 1. you can customize the interval between latitude/longitude ("lat/long") or UTM grid lines and print them on the map; if you use UTM grids (which are more symmetrical than lat/long and are based on meters rather than the more abstract degrees that lat/long provides), you can guesstimate your current position on the map based on your GPS readout vis-a-vis the grid lines printed on your map. 2. TOPO! automatically generates coordinates as you move your mouse around on the map and can even record and print out GPS way points directly on the map (printing the coordinates can, unfortunately, also obscure some contour lines). If you zoom all the way in on the map before placing your GPS waypoints, my experience is that TOPO's GPS waypoints are accurate to within a few meters! To avoid cluttering the map with GPS coordinates that obscure topographic detail, I simply print only the GPS waypoint mark (a small diamond icon) and record the coordinates to my GPS device.

              Coming from out of state, I initially used the National Geographic Trails Illustrated map to formulate a provisional route that I understood may or may not have worked (in fact, my original plan was way too ambitious and technically unrealistic). Once I had devised a route that had a pretty high probability of working and my trip was beginning to take shape, I bought TOPO! Utah to fine-tune my route.

              All that said, backpacking in Utah's red-rock country requires a lot of ground-truthing, because the proverbial 39-feet-high bands of rock, dry waterfalls and rock palisades that don't show up on a topo map can get you into serious trouble.

              That's what makes this forum so indispensable. I have relied heavily on the combined firsthand knowledge of people on this forum to formulate my route for an upcoming off-trail trip. I suggest you plan your route well before you embark for Zion and run it past the good folks on this forum to see if it's doable and find out what else you might need to know, such as the probability of finding water enroute.

              Best wishes,
              Michael


              --- In Zion_National_Park_Hiking@yahoogroups.com, Ray Fink <rkf@...> wrote:
              >
              > I have the National Geographic Trails Illustrated map of Zion NP, as
              > well as their maps for many other parks. I think they are generally
              > excellent, though of course they don't give you much detail as a 1:24K
              > USGS topo.
              >
              > The TOPO! National Parks software works pretty well for printing custom
              > maps, I use it as well as several other topographic software packages
              > for trip planning and custom maps. The USGS maps are still the gold
              > standard.
              >
              > Reid Preidhorsky on the Grand Canyon hikers list has done some
              > comparisons of printed maps from various sources. Check those out online
              > at http://reidster.net/trips/maps/
              >
              > -- Ray
              >
              >
              > jamesclark9 wrote:
              > > So I have been researching a bit on Amazon. I noticed National Geo has a hard copy topo map of Zion. The price seems to vary a bit when I look online. In addition National Geo has TOPO! National Geographic USGS Topographic Maps (Utah) and TOPO! National Parks electronic software that you can print maps from. Neither of these seem to have the extensive reviews that most Amazon.com products have. Does anyone have experience with these items?
              > >
              > > Thank You
              >
            • mojave_ben
              Haven t bought a hard copy map in years... Note that the great investigation Ried did (as referenced on the GC group) may be out of date - what s available
              Message 6 of 22 , May 1, 2010
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                Haven't bought a hard copy map in years... Note that the great investigation Ried did (as referenced on the GC group) may be out of date - what's available online now is pretty high res. There's plenty of free services - mapper.acme.com is one I use - that take provide a better user interface to the Terraserver data, but there are some places (northern Death Valley) where the maps are skewed as digitized and there is a bunch of error at the seams. Just find the area you want and print.

                There was a recent post over at the GC group, I think by Rich Rudrow, giving a URL for the USGS site which now has digital topos online and free.

                On the GPS thread 1) I've seen someone much more experienced than I make the "wrong datum" error, especially since many GPS units default to the 83 datum and the topos are printed in the NAD27 datum. You also of course really want to learn to use UTM - it's a 1 km grid and in UTM mode the least significant digit on your postion is in meters, so it is close to counting paces.

                But in any place with high relief, especially Zion canyons, your GPS isn't going to work well. And whatever you do, think about the question, what happens if your GPS dies? Can you still figure out where you are? Simply learning to visualize your position - i.e. visualize the terrain based on the contours - is a skill to develop.
              • mcooprec
                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:
                Message 7 of 22 , May 1, 2010
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                  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.

                  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.

                  Other useful navigation tools include your watch and the sun. Once you've determined your bearing to an intermediate destination, note the position of the sun and what time it is on your watch. Knowing roughly how fast you usually travel carrying a given load, maintain the sun's angle to your bearing and hike just shy of what you think it'll take in minutes to get to the next turn, obstacle or other significant feature along your route. Thus you reduce the need to be constantly taking field bearings and turning on your GPS (draining the batteries) enroute to the next move along your route.

                  Thanks for the links, Mohave Ben. Might come in handy next year, if I go somewhere outside of the 5 states I have topo software for.

                  Best,
                  Michael

                  --- In Zion_National_Park_Hiking@yahoogroups.com, "mojave_ben" <mojave_ben@...> wrote:
                  >
                  > Haven't bought a hard copy map in years... Note that the great investigation Ried did (as referenced on the GC group) may be out of date - what's available online now is pretty high res. There's plenty of free services - mapper.acme.com is one I use - that take provide a better user interface to the Terraserver data, but there are some places (northern Death Valley) where the maps are skewed as digitized and there is a bunch of error at the seams. Just find the area you want and print.
                  >
                  > There was a recent post over at the GC group, I think by Rich Rudrow, giving a URL for the USGS site which now has digital topos online and free.
                  >
                  > On the GPS thread 1) I've seen someone much more experienced than I make the "wrong datum" error, especially since many GPS units default to the 83 datum and the topos are printed in the NAD27 datum. You also of course really want to learn to use UTM - it's a 1 km grid and in UTM mode the least significant digit on your postion is in meters, so it is close to counting paces.
                  >
                  > But in any place with high relief, especially Zion canyons, your GPS isn't going to work well. And whatever you do, think about the question, what happens if your GPS dies? Can you still figure out where you are? Simply learning to visualize your position - i.e. visualize the terrain based on the contours - is a skill to develop.
                  >
                • TomJones
                  I have to respectfully disagree with you here, my dear Cooprec, on several points. ... TOM: GPS elevations are roughly twice as inaccurate as your lat-lon
                  Message 8 of 22 , 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
                  • mcooprec
                    Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted. As for
                    Message 9 of 22 , May 2, 2010
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                      Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted.

                      As for your assertion that contour-navigation is rarely useful in Utah, my admittedly limited experience would suggest that assertion is perhaps a little bit overstated (if specifically accurate for Zion). I used contour-navigation a few times traversing around the heads of basins in Capitol Reef NP a couple years ago. Our route was between Little Sand Flat and Spring Canyon (through the widest portion of the Park), and in order to avoid dropping down into rimrock-topped canyons we had to cross, we stayed high and traversed in arcs the basins at their heads. Our altimeters were extremely useful on this route. That said, I can see from the topos for my planned route in Zion that contour-navigation will only possibly be useful during a couple short stretches of terrain.

                      I agree generally with your observation about altimeters being sensitive to barometric pressure, Tom. (Btw, altimeters also use temperature change -- along with barometric pressure -- to determine change in elevation.) But some altimeters are amazingly almost immune to barometric pressure changes. My Suunto can go through an overnight storm and drift only 20 feet or so (dunno how it does this trick; Suunto are understandably cryptic about their proprietary technology)! My Casio altimeter, on the other hand, will drift 200 feet.

                      One way to mitigate an altimeter's sensitivity to barometric pressure fluctuations (I'm sure you already know this , Tom) is to first determine your elevation in camp by using your GPS and map. Then, the next morning while still in camp, recalibrate your altimeter to your known elevation if it drifted during the night. At key points along your route, such as at obvious landmarks and waypoints, recalibrate again (by referring to your topo). By always updating your altimeter reference, it will be dead-on accurate when you need to rely on it more than your other tools for an hour or so along your route.

                      Best to all,
                      Michael


                      --- In Zion_National_Park_Hiking@yahoogroups.com, "TomJones" <ratagonia@...> wrote:
                      >
                      > 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
                      >
                    • TomJones
                      ... TOM== Uh, no. Altimeters past the most primitive are temperature compensated ; which means a temperature sensor is used to correct the error in the
                      Message 10 of 22 , May 2, 2010
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                        --- In Zion_National_Park_Hiking@yahoogroups.com, "mcooprec" <coopermb@...> wrote:
                        >
                        > Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted.
                        >
                        > I agree generally with your observation about altimeters being sensitive to barometric pressure, Tom. (Btw, altimeters also use temperature change -- along with barometric pressure -- to determine change in elevation.) But some altimeters are amazingly almost immune to barometric pressure changes. My Suunto can go through an overnight storm and drift only 20 feet or so (dunno how it does this trick; Suunto are understandably cryptic about their proprietary technology)! My Casio altimeter, on the other hand, will drift 200 feet.
                        >
                        TOM==> Uh, no. Altimeters past the most primitive are "temperature compensated"; which means a temperature sensor is used to correct the error in the barometer due to changes of temperature. Using the lapse rate (the change in temperature due to altitude) would be impossible, as the lapse rate changes constantly with location and time - perhaps an Internet Link to more-detailed weather information that currently exists could use the lapse rate to determine elevation ;-)

                        That Suunto is one SMART altimeter. Perhaps it obtains GPS data and corrects itself. Or figures out when it is not moving and lets the barometer change. Or has gotten used in conditions where the morning barometric pressure is the same as the evening barometric pressure due to the nature of the storm.

                        Tom
                      • WB
                        Suunto s are temp compensated, Casio s are not. Temp compensation adjusts the thermal difference response in the altitude sensor itself. Warm up a Casio and
                        Message 11 of 22 , May 2, 2010
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                          Suunto's are temp compensated, Casio's are not. Temp compensation adjusts the thermal difference response in the altitude sensor itself. Warm up a Casio and the pressure sensor reads wildly different, not so (much) with the Suunto. Some Suunto manuals cover the procedure to manually compensate for the difference between the standard ISA temp/altitude and your local temps, this will allow you to remove almost all extraneous variation except the actual barometric pressure change due to weather.

                          Please note the difference between temp compensation in the sensor and temp compensation for differences between ISA and local conditions. Good altimeters will temp compensate in the sensor for the sensor temp, as far as I know (but I really am not up on these things) you have to compensate for ISA differences manually even on the good altimeters.

                          http://en.wikipedia.org/wiki/International_Standard_Atmosphere

                          The difference between the ISA temp/altitude charts and local is about 11% for typical Utah summer conditions. You can ballpark the elevation change you sense as being about 11% too little as reported by the sensor because the sensor is programmed to think the air is denser than it actually is so for each change of sensed 1K you actually changed elevation more than 1K. I'm going off memory for this so I think my numbers are correct but if anyone runs the numbers I'll stand to be corrected.

                          This is why your altimeter will read correctly at say the 8K level, be 100 feet off at 9K and descend back to 8K and be correct again. The altimeter thinks the column of air between 8 and 9K is a different density than it actually is. Winter mountaineers have their conditions more typically resemble the ISA conditions so this is less of a problem for them.

                          You can set your altimeter initially using the nearest airport and their ATIS if you're close to their elevation or know the difference. You can use the ATIS if you can hear it to track weather pressure changes. And as previously mentioned you can reset at known elevations.

                          If you compensate your altimeter correctly (and start with a temp compensated altimeter) then you will typically find the barometric variation to be the major variable. Unfortunately in the field away from easy radio or telephone access to ATIS close to you it is harder to compensate for this while on the move unless you are at known points in which case the altimeter is largely unneeded.

                          I don't find my altimeters very useful due to the above, but they're interesting. You can make them far more accurate by understanding the limitations of the device.

                          -Bill


                          On May 2, 2010, at 12:20 AM, mcooprec wrote:

                          >
                          > Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted.
                          >
                          > As for your assertion that contour-navigation is rarely useful in Utah, my admittedly limited experience would suggest that assertion is perhaps a little bit overstated (if specifically accurate for Zion). I used contour-navigation a few times traversing around the heads of basins in Capitol Reef NP a couple years ago. Our route was between Little Sand Flat and Spring Canyon (through the widest portion of the Park), and in order to avoid dropping down into rimrock-topped canyons we had to cross, we stayed high and traversed in arcs the basins at their heads. Our altimeters were extremely useful on this route. That said, I can see from the topos for my planned route in Zion that contour-navigation will only possibly be useful during a couple short stretches of terrain.
                          >
                          > I agree generally with your observation about altimeters being sensitive to barometric pressure, Tom. (Btw, altimeters also use temperature change -- along with barometric pressure -- to determine change in elevation.) But some altimeters are amazingly almost immune to barometric pressure changes. My Suunto can go through an overnight storm and drift only 20 feet or so (dunno how it does this trick; Suunto are understandably cryptic about their proprietary technology)! My Casio altimeter, on the other hand, will drift 200 feet.
                          >
                          > One way to mitigate an altimeter's sensitivity to barometric pressure fluctuations (I'm sure you already know this , Tom) is to first determine your elevation in camp by using your GPS and map. Then, the next morning while still in camp, recalibrate your altimeter to your known elevation if it drifted during the night. At key points along your route, such as at obvious landmarks and waypoints, recalibrate again (by referring to your topo). By always updating your altimeter reference, it will be dead-on accurate when you need to rely on it more than your other tools for an hour or so along your route.
                          >
                          > Best to all,
                          > Michael
                          >
                          >
                          > --- In Zion_National_Park_Hiking@yahoogroups.com, "TomJones" <ratagonia@...> wrote:
                          >>
                          >> 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
                          >>
                          >
                          >
                          >
                          >
                          > ------------------------------------
                          >
                          > If you would like to read posts on the net instead of taking email: Visit us on the web
                          > http://groups.yahoo.com/group/Zion_National_Park_Hiking/
                          >
                          > To stop mail:
                          > http://groups.yahoo.com/mygroups?sort=m&order=a&page=1&edit=1Yahoo! Groups Links
                          >
                          >
                          >
                        • mcooprec
                          The Suunto doesn t use GPS. Dunno how it stays so accurate. I backpack about 4 to 6 weeks each year and have been in many storms that blow in at different
                          Message 12 of 22 , May 2, 2010
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                            The Suunto doesn't use GPS. Dunno how it stays so accurate. I backpack about 4 to 6 weeks each year and have been in many storms that blow in at different times during the day or night, and my Suunto almost always seems to stay uncannily accurate (although a little less so now that it's several years old, which makes me think the sensor is beginning to degrade). It's highly unlikely that each of those storms produced the same barometric pressure each evening and following morning.

                            Best,
                            Michael

                            --- In Zion_National_Park_Hiking@yahoogroups.com, "TomJones" <ratagonia@...> wrote:
                            >
                            > --- In Zion_National_Park_Hiking@yahoogroups.com, "mcooprec" <coopermb@> wrote:
                            > >
                            > > Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted.
                            > >
                            > > I agree generally with your observation about altimeters being sensitive to barometric pressure, Tom. (Btw, altimeters also use temperature change -- along with barometric pressure -- to determine change in elevation.) But some altimeters are amazingly almost immune to barometric pressure changes. My Suunto can go through an overnight storm and drift only 20 feet or so (dunno how it does this trick; Suunto are understandably cryptic about their proprietary technology)! My Casio altimeter, on the other hand, will drift 200 feet.
                            > >
                            > TOM==> Uh, no. Altimeters past the most primitive are "temperature compensated"; which means a temperature sensor is used to correct the error in the barometer due to changes of temperature. Using the lapse rate (the change in temperature due to altitude) would be impossible, as the lapse rate changes constantly with location and time - perhaps an Internet Link to more-detailed weather information that currently exists could use the lapse rate to determine elevation ;-)
                            >
                            > That Suunto is one SMART altimeter. Perhaps it obtains GPS data and corrects itself. Or figures out when it is not moving and lets the barometer change. Or has gotten used in conditions where the morning barometric pressure is the same as the evening barometric pressure due to the nature of the storm.
                            >
                            > Tom
                            >
                          • mcooprec
                            Great explanation, Bill! Thanks for taking the time to write it up. Best, Michael
                            Message 13 of 22 , May 2, 2010
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                              Great explanation, Bill! Thanks for taking the time to write it up.

                              Best,
                              Michael

                              --- In Zion_National_Park_Hiking@yahoogroups.com, WB <listservew2@...> wrote:
                              >
                              > Suunto's are temp compensated, Casio's are not. Temp compensation adjusts the thermal difference response in the altitude sensor itself. Warm up a Casio and the pressure sensor reads wildly different, not so (much) with the Suunto. Some Suunto manuals cover the procedure to manually compensate for the difference between the standard ISA temp/altitude and your local temps, this will allow you to remove almost all extraneous variation except the actual barometric pressure change due to weather.
                              >
                              > Please note the difference between temp compensation in the sensor and temp compensation for differences between ISA and local conditions. Good altimeters will temp compensate in the sensor for the sensor temp, as far as I know (but I really am not up on these things) you have to compensate for ISA differences manually even on the good altimeters.
                              >
                              > http://en.wikipedia.org/wiki/International_Standard_Atmosphere
                              >
                              > The difference between the ISA temp/altitude charts and local is about 11% for typical Utah summer conditions. You can ballpark the elevation change you sense as being about 11% too little as reported by the sensor because the sensor is programmed to think the air is denser than it actually is so for each change of sensed 1K you actually changed elevation more than 1K. I'm going off memory for this so I think my numbers are correct but if anyone runs the numbers I'll stand to be corrected.
                              >
                              > This is why your altimeter will read correctly at say the 8K level, be 100 feet off at 9K and descend back to 8K and be correct again. The altimeter thinks the column of air between 8 and 9K is a different density than it actually is. Winter mountaineers have their conditions more typically resemble the ISA conditions so this is less of a problem for them.
                              >
                              > You can set your altimeter initially using the nearest airport and their ATIS if you're close to their elevation or know the difference. You can use the ATIS if you can hear it to track weather pressure changes. And as previously mentioned you can reset at known elevations.
                              >
                              > If you compensate your altimeter correctly (and start with a temp compensated altimeter) then you will typically find the barometric variation to be the major variable. Unfortunately in the field away from easy radio or telephone access to ATIS close to you it is harder to compensate for this while on the move unless you are at known points in which case the altimeter is largely unneeded.
                              >
                              > I don't find my altimeters very useful due to the above, but they're interesting. You can make them far more accurate by understanding the limitations of the device.
                              >
                              > -Bill
                              >
                              >
                              > On May 2, 2010, at 12:20 AM, mcooprec wrote:
                              >
                              > >
                              > > Thanks for the explanation, Tom. That makes sense, and I stand corrected. In any case, we both agree that GPS elevation readouts are not to be trusted.
                              > >
                              > > As for your assertion that contour-navigation is rarely useful in Utah, my admittedly limited experience would suggest that assertion is perhaps a little bit overstated (if specifically accurate for Zion). I used contour-navigation a few times traversing around the heads of basins in Capitol Reef NP a couple years ago. Our route was between Little Sand Flat and Spring Canyon (through the widest portion of the Park), and in order to avoid dropping down into rimrock-topped canyons we had to cross, we stayed high and traversed in arcs the basins at their heads. Our altimeters were extremely useful on this route. That said, I can see from the topos for my planned route in Zion that contour-navigation will only possibly be useful during a couple short stretches of terrain.
                              > >
                              > > I agree generally with your observation about altimeters being sensitive to barometric pressure, Tom. (Btw, altimeters also use temperature change -- along with barometric pressure -- to determine change in elevation.) But some altimeters are amazingly almost immune to barometric pressure changes. My Suunto can go through an overnight storm and drift only 20 feet or so (dunno how it does this trick; Suunto are understandably cryptic about their proprietary technology)! My Casio altimeter, on the other hand, will drift 200 feet.
                              > >
                              > > One way to mitigate an altimeter's sensitivity to barometric pressure fluctuations (I'm sure you already know this , Tom) is to first determine your elevation in camp by using your GPS and map. Then, the next morning while still in camp, recalibrate your altimeter to your known elevation if it drifted during the night. At key points along your route, such as at obvious landmarks and waypoints, recalibrate again (by referring to your topo). By always updating your altimeter reference, it will be dead-on accurate when you need to rely on it more than your other tools for an hour or so along your route.
                              > >
                              > > Best to all,
                              > > Michael
                              > >
                              > >
                              > > --- In Zion_National_Park_Hiking@yahoogroups.com, "TomJones" <ratagonia@> wrote:
                              > >>
                              > >> 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
                              > >>
                              > >
                              > >
                              > >
                              > >
                              > > ------------------------------------
                              > >
                              > > If you would like to read posts on the net instead of taking email: Visit us on the web
                              > > http://groups.yahoo.com/group/Zion_National_Park_Hiking/
                              > >
                              > > To stop mail:
                              > > http://groups.yahoo.com/mygroups?sort=m&order=a&page=1&edit=1Yahoo! Groups Links
                              > >
                              > >
                              > >
                              >
                            • Bernell Warner
                              Tom; Just wanted to let you know that I replaced the sling on the second to the last rep.  Also the, I think fifth rep with only one anchor now has two nice
                              Message 14 of 22 , Jul 7, 2010
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                                Tom;
                                Just wanted to let you know that I replaced the sling on the second to the last rep.  Also the, I think fifth rep with only one anchor now has two nice anchors on it.  Had a great time.  One of the rep's had chest deep pool, but nice and clear.
                                Bernell


                                From: TomJones <ratagonia@...>
                                To: Zion_National_Park_Hiking@yahoogroups.com
                                Sent: Sat, May 1, 2010 10:57:46 PM
                                Subject: [Zion_National_Park_Hiking] Re: GPS Routes

                                 

                                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


                              • Clay
                                Hey, Bernell, send me an email and I will send you some of my pictures I took on my last Parunuweap trip in MidSeptember 2008. Gary
                                Message 15 of 22 , Jul 7, 2010
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                                  Hey, Bernell, send me an email and I will send you some of my pictures I took on my last Parunuweap trip in MidSeptember 2008.
                                  Gary

                                  --- In Zion_National_Park_Hiking@yahoogroups.com, Bernell Warner <frogjump53@...> wrote:
                                  >
                                  > Tom;
                                  > Just wanted to let you know that I replaced the sling on the second to the last
                                  > rep.  Also the, I think fifth rep with only one anchor now has two nice anchors
                                  > on it.  Had a great time.  One of the rep's had chest deep pool, but nice and
                                  > clear.
                                  > Bernell
                                  >
                                  >
                                  >
                                  >
                                  > ________________________________
                                  > From: TomJones <ratagonia@...>
                                  > To: Zion_National_Park_Hiking@yahoogroups.com
                                  > Sent: Sat, May 1, 2010 10:57:46 PM
                                  > Subject: [Zion_National_Park_Hiking] Re: GPS Routes
                                  >
                                  >  
                                  > 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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