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

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  • Lauren R. Pacini
    Thanks for the input, Harlan. It is most helpful. Urban cayons are not ab bad there as one might juudge. There is wide green space and a boulevard on one
    Message 1 of 14 , Nov 30, 2004
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      Thanks for the input, Harlan. It is most helpful. "Urban cayons" are
      not ab bad there as one might juudge. There is wide green space and a
      boulevard on one side, and a road on the other. Further east is
      residential. going west (northwest) is into "canyonland" but the
      creek is culverted under a blanket of concrete there anyway. I will
      dust off my Legend and take a trek!

      Lauren


      --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Harlan Stockman"
      <hwstockman@y...> wrote:
      >
      > --- In GarminGPSMAP60C_60CS@yahoogroups.com, "Lauren R. Pacini"
      > <lrpacini@y...> wrote:
      > >
      > > Thanks, Harlan. Great questions and great thoughts. The
      topography
      > is
      > > mixed, ranging from flat and wide open to 25 foot deep gorge with
      > 80-
      > > 120 foot tall trees and dense secondary growth. The location is
      > > http://www.topozone.com/map.asp?
      > > z=17&n=4593718&e=450417&s=25&size=l&datum=nad83&layer=DRG25
      > >
      > > My goal is to use the GPS to set way points. We will record
      > > coordinates and elevation and use that in conjunction with
      compass
      > > readings, etc, to build the models.
      >
      > 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.
      >
      > Before you make a new purchase, take your old GPS, set it up so it
      > will display the "accuracy" or EPE, and watch the EPE as you walk
      > through the area, noting any correlations between trees and
      buildings
      > and bad readings. Note that the interference from deciduous trees
      > may be a lot worse in the summer, than it is now.
      >
      > Old-fashioned surveying from a fixed point would be a lot more
      > accurate if you are really interested in elevations a priori.
    • 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 2 of 14 , Dec 1, 2004
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        --- 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 3 of 14 , Dec 1, 2004
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          --- 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 4 of 14 , Dec 1, 2004
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            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 5 of 14 , Dec 1, 2004
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              --- 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 6 of 14 , Dec 1, 2004
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                --- 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 7 of 14 , Dec 2, 2004
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                  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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