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Re: Is the 60CS the Right Tool for the Job?

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  • pvs802
    ... You re overlooking something Harlan, the altimeter in the 60CS. This makes a big difference. Even if you lose GPS lock completely, the altimeter
    Message 1 of 14 , Dec 1, 2004
      --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Harlan Stockman"
      <hwstockman@y...> wrote:

      > Assuming I've got the right area, you are going to get hammered by
      > the tall trees and the "urban canyons" -- tall buildings. I expect
      > you'll rarely get WAAS, and probably won't even get a good fix a lot
      > of times. The topography looks pretty flat, so I'm guessing that
      > topography variations will be muddied by the +/- 50 feet error you
      > will occasionally have in Z position.

      You're overlooking something Harlan, the altimeter in the 60CS. This
      makes a big difference. Even if you lose GPS lock completely, the
      altimeter continues to work. So you may not get such an accurate
      position, but you should still get decent altimeter readings.

      I always have mine set to auto-calibrate, and I usually start by
      waiting until I get a good GPS lock, then calibrating the altimeter
      manually at a known height (at my house usually, before I set off,
      this gives the autocalibration plenty of time to settle down by the
      time I begin my walk). In use I find it to be surprisingly accurate,
      whenever I reach a summit or spot height the elevation is almost
      always within 2m of the height marked on the map (this is in the UK).

      Using an auto-calibrated altimeter gives much smoother elevation
      readings than GPS altitude, which is quite jumpy in comparison, but
      the latter prevents the gradual drift you get with an non
      auto-calibrated altimeter (by applying small gradual corrections).

      In my experience, the situation in deep forested valleys is a little
      different. I've just been comparing a recent 60CS tracklog with some
      older GPS12 tracklogs in the same area, and the new track looks more
      accurate than the older tracks, it lost lock fewer times and the
      errors were not so extreme when it did lose and regain lock.

      Having a correct altitude gives a better 2D position. A few days ago
      I turned my 60CS on and the initial position (as shown on the roadmap)
      was way off. I realised that the pressure had drifted since I'd last
      used it and the altitude was completely wrong, so I manually input the
      correct altitude and within a second or two the position jumped almost
      to the correct place. I presume therefore, that having an accurate
      altimeter reading will help provide a more accurate 2D position fix
      when reception is bad.

      However, the altimeter auto-cal has a "fast catch-up" feature, which
      means that when lock is regained after losing it, the altimeter
      rapidly recalibrates to the new GPS altitude, instead of making the
      usual small gradual corrections. Since the GPS altitude is usually
      way off when lock is first regained, this can throw the elevation
      right out when reception is bad.

      So in an intermittent lock situation, it's probably better to turn
      autocal off. That way the altimeter will be used independently of the
      GPS, so GPS elevation errors won't affect it. This should give smooth
      readings. If the altimeter elevation is still used to help compute
      the 2D position, this may result in a more accurate track (I don't
      know whether this is true or not, I'll have to test it).

      Used in this way the altimeter will of course drift with time, but
      over short periods it should remain quite accurate. Best to use it in
      stable high pressure conditions for the most accurate results, very
      little drift will occur in such conditions. Avoid windy days. Not
      only is the pressure lower when it's windy but I suspect that wind
      will also cause local pressure changes, depending on topography.

      In the track I was just studying the elevation started at 90m (true
      elevation 80m), rose to 180m (true elevation 150m), then returned to
      100m (back at the start position).

      So it started with a +10m error (I forgot to calibrate it before
      starting), rose to a +30m error (big jumps due to losing lock and fast
      catch-up errors), then returned to a +20m error (gradually settling
      back down due to autocal).

      Next time I do this walk I'll turn autocal off and manually calibrate
      the start position to 80m. It'll be interesting to see how much more
      accurate it is doing it this way.

      Anyway, the short answer is that any GPS with an altimeter should give
      more accurate results than one without IMO. Lauren could even buy a
      separate altimeter, but the advantages of autocal are well worth
      having in normal conditions.

      Paul
    • Harlan Stockman
      ... This ... I admit I hadn t even thought about the altimeter (I have a 60c), but I still question if a GPS is the tool for this job. There is not a lot of
      Message 2 of 14 , Dec 1, 2004
        --- In GarminGPSMAP60C_60CS@yahoogroups.com, "pvs802" <pvs1@w...>
        wrote:
        > You're overlooking something Harlan, the altimeter in the 60CS.
        This
        > makes a big difference. Even if you lose GPS lock completely, the
        > altimeter continues to work. So you may not get such an accurate
        > position, but you should still get decent altimeter readings.

        I admit I hadn't even thought about the altimeter (I have a 60c), but
        I still question if a GPS is the tool for this job. There is not a
        lot of elevation variation in the area he showed. 2 m is a lot
        compared to the cm precision one can get with traditional surveying.

        If you have extra time, do me this favor: send me two complete logs
        of your summit visits, where you spent some time on the summits, so
        the elevation variation can be seen as a function of time. Even gpx
        format will do. I'm curious about the drift. I ask this favor
        because there is in all people (including myself) a tendency to
        remember when the elevation was "right on," when often the log shows
        a lot of drift. Now, that drift may be gone in the altimeter version
        of the product.

        If you are willing to do this, I'll send you an e-mail address that
        will accept big files.
      • Lauren R. Pacini
        Great information. So much to think about. I appreciate all of the various experiences and input!! Lauren ... by ... expect ... lot ... you ... This ...
        Message 3 of 14 , Dec 1, 2004
          Great information. So much to think about. I appreciate all of the
          various experiences and input!!

          Lauren


          --- In GarminGPSMAP60C_60CS@yahoogroups.com, "pvs802" <pvs1@w...>
          wrote:
          >
          > --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Harlan Stockman"
          > <hwstockman@y...> wrote:
          >
          > > Assuming I've got the right area, you are going to get hammered
          by
          > > the tall trees and the "urban canyons" -- tall buildings. I
          expect
          > > you'll rarely get WAAS, and probably won't even get a good fix a
          lot
          > > of times. The topography looks pretty flat, so I'm guessing that
          > > topography variations will be muddied by the +/- 50 feet error
          you
          > > will occasionally have in Z position.
          >
          > You're overlooking something Harlan, the altimeter in the 60CS.
          This
          > makes a big difference. Even if you lose GPS lock completely, the
          > altimeter continues to work. So you may not get such an accurate
          > position, but you should still get decent altimeter readings.
          >
          > I always have mine set to auto-calibrate, and I usually start by
          > waiting until I get a good GPS lock, then calibrating the altimeter
          > manually at a known height (at my house usually, before I set off,
          > this gives the autocalibration plenty of time to settle down by the
          > time I begin my walk). In use I find it to be surprisingly
          accurate,
          > whenever I reach a summit or spot height the elevation is almost
          > always within 2m of the height marked on the map (this is in the
          UK).
          >
          > Using an auto-calibrated altimeter gives much smoother elevation
          > readings than GPS altitude, which is quite jumpy in comparison, but
          > the latter prevents the gradual drift you get with an non
          > auto-calibrated altimeter (by applying small gradual corrections).
          >
          > In my experience, the situation in deep forested valleys is a little
          > different. I've just been comparing a recent 60CS tracklog with
          some
          > older GPS12 tracklogs in the same area, and the new track looks more
          > accurate than the older tracks, it lost lock fewer times and the
          > errors were not so extreme when it did lose and regain lock.
          >
          > Having a correct altitude gives a better 2D position. A few days
          ago
          > I turned my 60CS on and the initial position (as shown on the
          roadmap)
          > was way off. I realised that the pressure had drifted since I'd
          last
          > used it and the altitude was completely wrong, so I manually input
          the
          > correct altitude and within a second or two the position jumped
          almost
          > to the correct place. I presume therefore, that having an accurate
          > altimeter reading will help provide a more accurate 2D position fix
          > when reception is bad.
          >
          > However, the altimeter auto-cal has a "fast catch-up" feature, which
          > means that when lock is regained after losing it, the altimeter
          > rapidly recalibrates to the new GPS altitude, instead of making the
          > usual small gradual corrections. Since the GPS altitude is usually
          > way off when lock is first regained, this can throw the elevation
          > right out when reception is bad.
          >
          > So in an intermittent lock situation, it's probably better to turn
          > autocal off. That way the altimeter will be used independently of
          the
          > GPS, so GPS elevation errors won't affect it. This should give
          smooth
          > readings. If the altimeter elevation is still used to help compute
          > the 2D position, this may result in a more accurate track (I don't
          > know whether this is true or not, I'll have to test it).
          >
          > Used in this way the altimeter will of course drift with time, but
          > over short periods it should remain quite accurate. Best to use it
          in
          > stable high pressure conditions for the most accurate results, very
          > little drift will occur in such conditions. Avoid windy days. Not
          > only is the pressure lower when it's windy but I suspect that wind
          > will also cause local pressure changes, depending on topography.
          >
          > In the track I was just studying the elevation started at 90m (true
          > elevation 80m), rose to 180m (true elevation 150m), then returned to
          > 100m (back at the start position).
          >
          > So it started with a +10m error (I forgot to calibrate it before
          > starting), rose to a +30m error (big jumps due to losing lock and
          fast
          > catch-up errors), then returned to a +20m error (gradually settling
          > back down due to autocal).
          >
          > Next time I do this walk I'll turn autocal off and manually
          calibrate
          > the start position to 80m. It'll be interesting to see how much
          more
          > accurate it is doing it this way.
          >
          > Anyway, the short answer is that any GPS with an altimeter should
          give
          > more accurate results than one without IMO. Lauren could even buy a
          > separate altimeter, but the advantages of autocal are well worth
          > having in normal conditions.
          >
          > Paul
        • pvs802
          ... Perhaps, but doesn t traditional surveying involve a lot of time, care and maths? I thought this was just a bunch of kids we were talking about, how
          Message 4 of 14 , Dec 1, 2004
            --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Harlan Stockman"
            <hwstockman@y...> wrote:

            > I admit I hadn't even thought about the altimeter (I have a 60c), but
            > I still question if a GPS is the tool for this job. There is not a
            > lot of elevation variation in the area he showed. 2 m is a lot
            > compared to the cm precision one can get with traditional surveying.

            Perhaps, but doesn't traditional surveying involve a lot of time, care
            and maths? I thought this was just a bunch of kids we were talking
            about, how accurate do they need it to be?

            > If you have extra time, do me this favor: send me two complete logs
            > of your summit visits, where you spent some time on the summits, so
            > the elevation variation can be seen as a function of time.

            Okay.

            > Even gpx format will do.

            I save all my data in OziExplorer format. I have GPS Utility
            Freeware, which can export in many different formats. Which would
            you prefer?

            > I'm curious about the drift. I ask this favor
            > because there is in all people (including myself) a tendency to
            > remember when the elevation was "right on," when often the log
            > shows a lot of drift. Now, that drift may be gone in the altimeter
            > version of the product.

            It's not gone completely, the elevation still "wobbles", but over a
            much smaller range, typically 2-3m.

            > If you are willing to do this, I'll send you an e-mail address that
            > will accept big files.

            Okay, but the files are not big anyway.

            My local city has lots of spot heights marked at road junctions, so
            the first thing I did when I got my 60CS was to do a couple of "spot
            height" drives, visiting a number of spot heights, stopping the car
            briefly at each, and marking a waypoint. The results are below.

            In both cases I began by waiting until I got a decent lock, then
            manually calibrating the altimeter to my home altitude of 71m. Some
            waypoints weren't marked in exactly the right spot (it wasn't always
            obvious where the spot height was supposed to be). Being mounted in
            the car the GPS was probably about a metre above the ground, so 1m
            should be subtracted from each reading, in which case the results seem
            to have been slightly low both times. I've read that autocal really
            needs about 1/2 hour to properly settle down, but I didn't wait for it
            to do that.

            Drive 1
            Spot height - waypoint elevation - difference
            38 37 -1
            27 24 -3 (marked whilst moving)
            58 56 -2
            62 62 0
            39 40 +1
            18 17 -1
            19 19 0
            24 24 0
            13 12 -1
            43 42 -1
            34 35 +1
            46 45 -1
            73 71 -2
            87 85 -2
            74 74 0
            49 49 0
            71 70 -1 (Home)

            Drive 2
            Spot height - waypoint elevation - difference
            151 150 -1
            122 123 +1
            131 131 0
            134 134 0
            129 129 0
            122 120 -2
            107 106 -1
            091 092 +1
            071 070 -1 (Home)

            Paul
          • Harlan Stockman
            ... but ... a ... surveying. ... care ... I learned how to use a plane table and alidade in about 30 minutes; ceratinly there are more precise instruments
            Message 5 of 14 , Dec 1, 2004
              --- In GarminGPSMAP60C_60CS@yahoogroups.com, "pvs802" <pvs1@w...>
              wrote:
              >
              > --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Harlan Stockman"
              > <hwstockman@y...> wrote:
              >
              > > I admit I hadn't even thought about the altimeter (I have a 60c),
              but
              > > I still question if a GPS is the tool for this job. There is not
              a
              > > lot of elevation variation in the area he showed. 2 m is a lot
              > > compared to the cm precision one can get with traditional
              surveying.
              >
              > Perhaps, but doesn't traditional surveying involve a lot of time,
              care
              > and maths? I thought this was just a bunch of kids we were talking
              > about, how accurate do they need it to be?

              I learned how to use a plane table and alidade in about 30 minutes;
              ceratinly there are more precise instruments available. Since there
              is a direct transfer to an image on the plane table, there is not
              much math required. The worst thing about the system is that the
              paper gets grungy from sweat. But nowadays, it might be pretty hard
              to get a plane table and alidade.


              > > If you have extra time, do me this favor: send me two complete
              logs
              > > of your summit visits, where you spent some time on the summits,
              so
              > > the elevation variation can be seen as a function of time.
              >
              > Okay.
              >
              > > Even gpx format will do.
              >
              > I save all my data in OziExplorer format. I have GPS Utility
              > Freeware, which can export in many different formats. Which would
              > you prefer?

              GPX is best, since it is basically a text file. There is also a
              garmin format that is basically text. Anything that you can read
              with a text editor (e.g. wordpad), yet still has elevations, is good.
              Maany programs will allow you to save in csv format which is also
              text.

              I can also read most raw Garmin formats, such as mps.

              > > I'm curious about the drift. I ask this favor
              > > because there is in all people (including myself) a tendency to
              > > remember when the elevation was "right on," when often the log
              > > shows a lot of drift. Now, that drift may be gone in the
              altimeter
              > > version of the product.
              >
              > It's not gone completely, the elevation still "wobbles", but over a
              > much smaller range, typically 2-3m.

              Thanks for the info about the benchmarking.

              I looked up the logs from a hike where a friend took a 60cs, and I
              took a 60c. He took two waypoints at the same station 37 seconds
              apart, and they differed by 2.3 feet. He had a 500 point "saved"
              track, so the inherent averaging may have wiped out the variation in
              the track itself. He couldn't enter a known elevation at the start of
              the hike. We gained about 3000', to end up at a summit surveyed at
              10161' (low estimated accuracy -- this was one of the geodetic
              surveys T points, which are really done from aerial photos, relative
              to nearby high-accuracy benchmarks). The 60cs gave the elevation as
              ~10174'. The 60c gave the elevation as ~10190', with a variation of
              about 6' over the time I was on the summit (15 minutes). However, I
              was getting decent WAAS corretions, as the mountaintop was completely
              open.
            • Harlan Stockman
              Here is my first look at the data. Thanks very much to Paul S for supplying the gpx files; it s rare anyone responds with so much detail and frankness. I just
              Message 6 of 14 , Dec 2, 2004
                Here is my first look at the data. Thanks very much to
                Paul S for supplying the gpx files; it's rare anyone responds with so
                much detail and frankness.

                I just had a quick chance to go over the Moel Feity log. This was a
                simple loop, with starting and end points within 0.65 meters. Paul
                notes this may not be a good altimeter test, since it was quite windy
                the day of the track.


                A summary of elevation vs. walk time is here:


                http://hwstock.org/60cs/MoelF.gif

                ...and here is a "zoom" view of the walk at top:
                http://hwstock.org/60cs/MoelF_zoom.gif

                The latter shows the elevation flitting around within ~ 2 meter band,
                with one flit of 4 meters on a time scale of seconds. The odd
                discretization of elevation points probably reflects precision and
                unit conversions from the native digital format. It isn't clear if
                this track involves some garmin time-averaging, as the number of
                points is rather low.

                Paul and I both noticed something a bit odd: the reported difference
                in the starting and ending elevations -- presumably within a meter x-
                y -- of about 3-4 meters.

                My first observations:
                1) I'm impressed that any barometric altimeter is that accurate. I
                normally want an altimeter for cases when I'm climbing a chute, and
                the view of the sky is so bad that my GPS loses sync. Most watch-
                type altimeters seem to be off by up to 60 meters in such
                circumstances.

                2) Nonetheless, I might be a bit wary of trusting the 60cs to be
                reproducible to less than 3 meters elevation, over a period of 1.5
                hours, or even over seconds.
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