Puzzled

Discussion in 'General Fitness' started by Baz, Nov 22, 2005.

  1. Baz

    Baz Guest


    >
    > It would be ideal if you could download the recorded track directly from
    > the GPS into another program (such as the freeware GPS TrackMaker
    > http://www.gpstm.com/ ) to eliminate the possibility that MM is doing
    > something funny to the data when it downloads it from the GPS.
    >


    I got gps trackmaker and downloaded the track and route from my geko,
    exactly the same distances in trackmaker as in MM, but something else
    showed up which was not shown im MM, a 1.9 mile section of track at the
    beginning of the track traveled at 308 mph, I think this must be caused
    by the geko getting a false sat fix, and then having to correct itself?
     


  2. In article <[email protected]>, Baz
    <[email protected]> writes
    >
    >> It would be ideal if you could download the recorded track directly
    >>from the GPS into another program (such as the freeware GPS TrackMaker
    >>http://www.gpstm.com/ ) to eliminate the possibility that MM is doing
    >>something funny to the data when it downloads it from the GPS.
    >>

    >
    >I got gps trackmaker and downloaded the track and route from my geko,
    >exactly the same distances in trackmaker as in MM, but something else
    >showed up which was not shown im MM, a 1.9 mile section of track at the
    >beginning


    Taking that 1.9 miles off gives you the distance of your planned route
    almost exactly.

    > of the track traveled at 308 mph, I think this must be caused by the
    >geko getting a false sat fix, and then having to correct itself?


    That can often happen when you first turn a GPS on and it is made worse
    by moving a long distance from where it was last used or a significant
    elevation change since last used.

    If it has been switched off for over 2 hours (Garmin - other
    manufacturers may have a different time - up to 4 hours) the GPS makes
    some assumptions to get the initial fix and these can lead to large
    errors if there is a significant change from where it is last used but
    as you saw they generally correct themselves quite quickly.

    I recommend that people let their unit settle for at least a minute
    after turning it on when it has not been used for 2 hours or more. That
    way it will usually not give large errors. If you want to you can clear
    the track memory then and avoid such errors...

    --

    Dominic Sexton
     
  3. Dominic Sexton wrote:
    > Baz writes


    >> but something else
    >> showed up which was not shown im MM, a 1.9 mile section of track at
    >> the beginning


    It's not usually as much as that, but it's not unusual, especially if you
    switch your GPS on in a bad reception area, such as under trees or
    surrounded by buildings. In the past, if I knew I'd be starting a walk in
    such a location, I'd make a point of turning the GPS on in the car a mile or
    two before I got there, to give it a chance to get a decent lock in advance.

    And as I said, I never turn track recording on until I start the walk.

    > If it has been switched off for over 2 hours (Garmin - other
    > manufacturers may have a different time - up to 4 hours) the GPS makes
    > some assumptions to get the initial fix and these can lead to large
    > errors if there is a significant change from where it is last used but
    > as you saw they generally correct themselves quite quickly.
    >
    > I recommend that people let their unit settle for at least a minute
    > after turning it on when it has not been used for 2 hours or more.
    > That way it will usually not give large errors. If you want to you
    > can clear the track memory then and avoid such errors...


    Nowadays I do things differently, due to having an altimeter in my GPS. I
    always start by switching it on in the car at home, waiting for it to
    settle, then manually calibrating the altitude to 71m. I find it better to
    do that at home where I know the altitude with certainty, rather than
    estimate it at the start of the walk (where it can often be difficult to
    ascertain it precisely from the map, especially if it's a flat area), or
    worse still, calibrate it from the GPS altitude.

    Setting it in the car also has the advantage that it gives the
    auto-calibration feature time to settle down, so it should be fairly
    accurate by the time I reach my start point. Comparing it to the contours
    in my GPS seems to validate that (although they can't always be trusted).

    However, it's not uncommon for there to be a discrepancy between the start
    and end altitude of my walks, and I suspect this may have something to do
    with the car being a bit of a pressure bubble. I usually make a point of
    leaving a couple of windows open if the weather permits, to help alleviate
    this effect, but still, using an altimeter in a moving car can't be that
    reliable, can it? Even with the windows open there'd still be a pressure
    difference behind the windscreen, wouldn't there?

    Anyway, just prior to starting my walk I reset the stats (to clear the data
    from the drive) and turn the track recorder on, then turn it off again at
    the end of the walk. If I choose to use the GPS for the return drive I'll
    jot down the walk stats on a scrap of paper first.

    When I get back home, the altimeter almost always reads 70-72m.

    Paul
     
  4. Bitstring <[email protected]>, from the wonderful
    person Baz <[email protected]> said
    >
    >> It would be ideal if you could download the recorded track directly
    >>from the GPS into another program (such as the freeware GPS
    >>TrackMaker http://www.gpstm.com/ ) to eliminate the possibility that
    >>MM is doing something funny to the data when it downloads it from the GPS.
    >>

    >
    >I got gps trackmaker and downloaded the track and route from my geko,
    >exactly the same distances in trackmaker as in MM, but something else
    >showed up which was not shown im MM, a 1.9 mile section of track at the
    >beginning of the track traveled at 308 mph, I think this must be caused
    >by the geko getting a false sat fix, and then having to correct itself?


    Yep, that's the "really stupid point when you turn the GPS on and it
    gets an initial lock" I was talking about, which most software is smart
    enough to dump.

    --
    GSV Three Minds in a Can
    Contact recommends the use of Firefox; SC recommends it at gunpoint.
     
  5. Baz

    Baz Guest

    Dominic Sexton wrote:
    > In article <[email protected]>, Baz
    > <[email protected]> writes
    >>
    >>> It would be ideal if you could download the recorded track directly
    >>> from the GPS into another program (such as the freeware GPS
    >>> TrackMaker http://www.gpstm.com/ ) to eliminate the possibility that
    >>> MM is doing something funny to the data when it downloads it from the
    >>> GPS.
    >>>

    >>
    >> I got gps trackmaker and downloaded the track and route from my geko,
    >> exactly the same distances in trackmaker as in MM, but something else
    >> showed up which was not shown im MM, a 1.9 mile section of track at
    >> the beginning

    >
    > Taking that 1.9 miles off gives you the distance of your planned route
    > almost exactly.
    >
    >> of the track traveled at 308 mph, I think this must be caused by the
    >> geko getting a false sat fix, and then having to correct itself?

    >
    > That can often happen when you first turn a GPS on and it is made worse
    > by moving a long distance from where it was last used or a significant
    > elevation change since last used.
    >
    > If it has been switched off for over 2 hours (Garmin - other
    > manufacturers may have a different time - up to 4 hours) the GPS makes
    > some assumptions to get the initial fix and these can lead to large
    > errors if there is a significant change from where it is last used but
    > as you saw they generally correct themselves quite quickly.
    >
    > I recommend that people let their unit settle for at least a minute
    > after turning it on when it has not been used for 2 hours or more. That
    > way it will usually not give large errors. If you want to you can clear
    > the track memory then and avoid such errors...
    >

    reception was pretty poor probably due to a dense line of trees about 30
    meters to the south, track memory was cleared as soon as I switched unit
    on, and I let it settle down for maybe 5 mins on the car roof before
    starting the walk
     
  6. In article <[email protected]>, Baz
    <[email protected]> writes
    >reception was pretty poor probably due to a dense line of trees about
    >30 meters to the south, track memory was cleared as soon as I switched
    >unit on, and I let it settle down for maybe 5 mins on the car roof
    >before starting the walk


    If you cleared the track after letting it settle down you most probably
    would not have had the error in the track.

    --

    Dominic Sexton
     
  7. Rudi Winter

    Rudi Winter Guest

    Adrian Godwin <[email protected]> wrote:
    > Baz <[email protected]> wrote:
    >> Yesterday I planned a route in Derbyshire using Memory Map, the route
    >> was 8.3 miles, I then uploaded the route to my Garmin Geko and went and
    >> walked the route.
    >> At the end of the walk the trip odometer on the Geko read 8.1 miles, but
    >> the strange thing was, when I got home I downloaded the track to Memory
    >> Map, MM showed the track to be 10.1 miles even though it followed the
    >> route exactly.

    >
    > All those measurements are different, and the errors between them
    > can be quite large.
    >
    > The route plan is the sum of all the straight-line segments you
    > drew on the map.


    But an error of 25% sounds a bit over the top even taking into account
    these errors. If you remove any obvious deviations when the receiver
    gets confused (say, in a forest), and Baz said there weren't any such
    effects, then the error should only be a few percent. It's never been
    more than that for me, anyway (Geko 201 and Tracklogs).

    My biggest experiment was an eight-day 200km walk this summer. The
    difference between planned mileage in Tracklogs, odometer, and mileage
    of the tracklog transferred back into Tracklogs were all within 2%.

    Altitudes are a different matter, though.
    --
    Rudi Winter, Aberystwyth, Wales
     
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